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PostPosted: Sat Jul 07, 2007 11:44 am 
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I'm experimenting with cathode resistor values on my el84 my dc-30 clone.
The dc-30 schematic has high low power switch. on high power it has a 62ohm cathode resistor and on low power adds another 68 ohms to cathode resistor value.

Hence on low power the cathode resistor value is doubled - the effect I notice is that it has much less headroom and doesn't sound as nice on full power. I dunno if it is this alone that causes it.

I noticed 60's ac-30 schemo has a sole 50 ohm cathode resistor. Will a lower cathode resistor value on the el84 give a cleaner headroom?
If I wire low power switch so it has a 60 ohm cathode resistor will this give more clean headroom at lower power?


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PostPosted: Sun Jul 08, 2007 5:07 pm 
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Hi Daz

Did you notice that on the right hand side of that original factory circuit for the 1960 AC30 there is a list of modifications....and near the bottom of the list, it says 'R24 was 80 Ohms'? It looks like that change may have been made in 1963. It says something about that in the Vox Story book, but it's almost 6 a.m. at the moment and i'm not up to searching for that bit of info in there until i've had some sleep.

The lower the value of the cathode resistor, the more current is going to flow. I have just got my DC30 orientated amp going (will be posting about that soon) and at first it was just TOO gain-y. I learnt a lot 'scoping the signal all through the stages. Basically what i discovered is that both channels delivered a perfectly clean signal to the input of the P/I. It is the P/I that overloads when it's input reaches a certain size. There was quite a discrepancy between the gain of the two channels, so i cooled down the EF86 channel to get it closer to the TB channel (on my amp i have summed the two channels and they feed one side of the P/I as there is going to be a valve effects loop between the summing point and the P/I).
Cooling off the EF86 channel allows me to get the volume knob up further before things get too crazy...i.e. before it overdrives the P/I.

I am running my power valves in pairs, with a standby switch in the cathode of each pair. So each pair has it's own cathode resistor and bypass cap. I have 135 Ohms on each pair at the moment, and although there is no red-plating, i know i could still run the EL84's a little cooler. (That obviously equates to about 67 Ohms for the 4 power valves).

Don't forget that changing the cathode resistor changes the current flowing through the valves, therefore the Plate voltage will change. This can lead to a bit of juggling to get the current flow right in relation to the plate voltage. (A lot of people speak of what is the 'right' current, but don't mention the plate voltage). I use the Bias Charts in the Dan Torres book, they give current settings for given plate voltages.

VOLTAGE.......... CLASS A/B.............HIGH A/B..........CLASS A

300..................36.1.........................32.8................. 38.0
310..................25.2........................31.7..................36.8
320..................24.4........................30.8................. 35.6
330..................23.6.........................29.8.................34.5
340..................22.9.........................28.9.................33.5
350..................22.3.........................28.1.................32.6


I assume these figures to be the plate current only, and not include the screen current.

Traditionally, the 2 power-valve mode sounds almost as loud as the 4 power valve mode but the full power operation is capable of the greater clean headroom.....this is why a 15-20 watt 2 X EL84 amp will keep up with a drum kit when playing lead, but try to do some clean rhythm work and forget it. Kick in the other pair of tubes and it won't seem a whole lot louder but suddenly there is headroom available for the clean sounds to keep up with the drums.

This is my first experience with an amp of this sensitivity (i've built lots of amps) and i've usually been a kinda player who runs the guitar wide open as much as possible. This amp is definitely something to be driven from the guitars controls. Turning down cleans things up beautifully, winding up the knob gets things really happening. But what i understand now is that even when the guitar is wide open, it is not overdriving the EF86 or the 12AX7's in the TB channel, they stay clean, but there is a point where their output becomes too much for the P/I. Regardless of what you do at the power valve stage, the overdrive is going to be there from the PI. On a scale of 10, i could only turn the EF86 channel up to 3 before the wave shape distorted, and the TB channel up to 5. Now i have matched the '86 ch to the TB and it's got more useable cleans, but still PLENTY of ferociousness on tap.

Ooops, sorry, this is what these all-nighters can do....but i hope there might be something in my very recent experiences that could be of some help. If you have a 'scope, dummy load and signal generator, or can get to someone who has, it's well worth a look to see what's happening and at what point things start to overdrive in which parts of the circuit.

I'll try to get back with what the Vox book has to say about why they changed that cathode resistor value.

neil.

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PostPosted: Sun Jul 08, 2007 7:07 pm 
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thanks neil for the great info. I noticed you are from Melbourne. I'm in Sydney myself.
I guess my gripe with the the dc-30 is balancing volume levels between channels. I like nice crunch with the ef86 but is too loud in comparison the 12ax7 side which I like to be clean with a hint of breakup. I find it currently alittle unpleasant - it has a harsh breakup even at lower gain levels. Maybe I need to check my preamp valves (I just put in some new el84)

I have lowered the cathode bias bypass cap from 220uf to 22uf and that made it less fizzy gainwise, and less flabby and wooly on the bottom end of the amp.
I also lowered the cathode cap on the 86 side to 10uf as I found the amp "chokes" up at higher gain levels and this helped in combination with the above.

Ideally I'd like to clean up the 12ax7 more, I tried taking a gain stage out of v1 but I found alittle thin and less chimey doing that.
I also lowered the cathode cap of v2 to about 1-2uf.


Any other thoughts/ideas would be appreciated.


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PostPosted: Mon Jul 09, 2007 5:29 am 
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Hi

Ok...i have an idea for you, and in this respect you're better off than me because you have each channel feeding a separate input on the PI whereas i have summed my 2 channels together.

You see where the 12AX7 from the TB channel feeds the PI? There is a 220K resistor in series from the treble pot wiper going to the 0.01 cap and then into the PI. At that same junction (220K/0.01 cap) there is a 100K resistor to ground. The 220K/100K resistors form a voltage divider between the signal and ground, kinda like a pre-set pot. What you could do is create a voltage divider like this on the EF86 channel.

My thoughts would be....remove the 470K resistor between the EF86 volume pot wiper and ground. Connect a pot temporarily between the volume pot wiper and ground, using the outer pot terminals. (maybe make the pot 250K or 500K). Then connect the wiper of this temporary pot to the 0.01 cap that feeds the EF86 into the PI input. Now experiment with the settings until you find a position on the temporary pot that gets the EF86 channel working the way you like it. THEN....carefully disconnect this added pot and measure the resistance from the wiper to the outside terminals and write them down. Then find 2 resistors as close to those values as possible and install them, but make sure you get them the right way round.

On my amp, I have a 220K in series with the signal, coming off the volume pot wiper (470K to ground in schematic is removed) and feeding the 0.01 PI input cap, and at that junction (220K/0.01) i have a 220K to ground. So basically a 50/50 divider. It works well but as i say, you may want to experiment more, hence the idea of the temporary pot to work out the values, and it is a lot quicker that soldering resistors in and out.

Sound like a plan?

Yes I'm in Melbourne, i was reading a post where a guy down here (Barry M?) said you'd been down for a visit and had a listen to an EF86 channel he'd added to a Lightning or something.....he's not the guy who has the amp shop in Williamstown is he? (Blank amps i think it is...i haven't been there, even though it's only a few K's down the road, but my mate has and i've seen the website).

It's interesting that there seems to be a resurgence of interest in the EF86. I am kinda smiling because i have been using it for years, i've made a heap of Vox AC4's, icluding some that are the driver amps in spring reverb units, and i designed and have built several microphone pre-amps using our trusty pentode friend. I also have 3 Leak hi-fi preamps here and they all use the '86 as well. The Vox AC4 is a great little amp and can be powered by 2 readily available tranformers available from Dick Smiths and Jaycar, turned back-to-back, and i never understand why it has not been more widely championed). I don't complain though, i just sit back and enjoy the great tone).

Oh yeah....is there any reason you posted this here rather than the 30 Watt section? I posted there a week or so ago, and i have noticed it seems kinda....well....quiet... in there regarding the ammount of posts compared to the other sections. Maybe that's why you posted in here....where there seems to be more action...?....

Let me know what you think of the voltage divider idea....


cheeers, neil.

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PostPosted: Mon Jul 09, 2007 5:33 am 
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Oh yeah...i forgot to mention....i changed my EF86 components back to the Vox values...220K plate resistor instead of 330K, and 1Meg sereen resistor instead of 2.2 Meg......but i still had to create the voltage divider i speak of in my post above......

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PostPosted: Mon Jul 09, 2007 7:17 am 
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thanks for the detailed response neil.
Sounds like you've had alot of experience dealing with valve circuits.

You should chat & meet Barry Marie, he is a really nice guy, he's built plenty of amps.

What experiences have you had with ef86 and microphonics esp. in combo amps? You should look at e80f valve, same as a ef86, more robust, and sound nicer too.

Thanks for your voltage divider idea on the ef86 side, I will give it a go when I get a chance and let you know
Do you have any thoughts on getting more clean headroom from the 12ax7 side? tried many things not to my liking.

Yeah posted here cause 30watt section is pretty quiet.

Yes my wife's family are all in melbourne so I do make scheduled visits down there.



Regards
daz


Last edited by daz on Mon Jul 09, 2007 9:24 am, edited 1 time in total.

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PostPosted: Mon Jul 09, 2007 7:32 am 
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hey guys'
haven't dealt with a EF90. More robust always sounds good. Designs witat particular tube flying around?
Thanks
Jac


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PostPosted: Mon Jul 09, 2007 1:48 pm 
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Hi again

Re. microphonics with the EF86.....

I had a Selmer Thunderbird up until recently, and that is a 50 watt combo with 2 x 12" speakers. Each of it's 2 channels has an EF86 and a 12AX7. Even though they were built in the same era as the Voxes, the Selmers were ahead in some engineering aspects, in this case...shock mounted valve sockets. I used to get that thing shaking and didn't have any problems with microphonics.
The DC30-ish amp i have just built is a head, and i never usually put a head on top of a speaker cabinet (but i did use a nice shock-mounted socket anyway). Some of the AC4 circuits i have built (the AC4 uses only an EF86 as a pre, straight into a single EL84) have been in little combo's, sometimes even with a 12" spkr, but with only 4 (albeit a LOUD 4) watts, chasis shaking and microphonics have never been a problem. And i do like to dial up the more 'thumpy' end of the tone spectrum, too.
Perhaps i've just been lucky........i've dismanted a few old valve tape recorders and record players either for parts or converting or restoring, and the better ones have often had good shock-mounted sockets for the EF86's.


Thoughts for cooling down the 12AX7 (Top Boost) channel.....

Okay Daz....i'm gonna quote myself from an earlier post here (yeah, i know, but it's not like THAT, really, it's just i wrote it once already and i can save some time....hehe)......

"You see where the 12AX7 from the TB channel feeds the PI? There is a 220K resistor in series from the treble pot wiper going to the 0.01 cap and then into the PI. At that same junction (220K/0.01 cap) there is a 100K resistor to ground. The 220K/100K resistors form a voltage divider between the signal and ground, kinda like a pre-set pot. What you could do is create a voltage divider like this on the EF86 channel. "

Disgregard the last line of that, as yes, indeed that's a way to deal with the EF86 level....but the point this time is what i wrote about the 12AX7 channel....you already have a voltage divider built in there....the 220K resistor coming from the treble pot wiper, and then the 100K resistor that joins the 220K/0.01 cap and goes to ground.
So....you could think of that as a 320K pot (220K + 100K) from the treble pot wiper to ground, and it's pre-set at about 1/3 of the way up. Almost a pre-set master volume. If you replaced the 100K resistor with a piece of wire, you'd short the audio path directly to ground and get no signal into the P/I. So methinks...the answer lies somewhere in between 0 Ohms and 100K. Or you could INCREASE the 220K resistor and leave the 100K as is. Or change both resistor values to create a new ratio. The simplest way would be to replace the 100K with something lower...work your way down through the values, OR...even pull the 100K out and tack in a 100K pot there and find the setting that works for you and then read the resistance of the setting and install the closest value resistor. Listen, too, when you make changes here as there could be some discernable tone changes, but i suspect if they are noticeable, they will only be slight.

I spose you could try a 12AU7 in place of one of the two 12AX7's?....

BTW....if it was me, i'd restore the cathode caps back to the original specs, and any other changes, to keep the original tone once i started lowering the gain using these methods. I'd want the best tone available, just less of it.....but then again....and this is where it can get tedious....lose a little here and a little there might work better, and the only way to really know is try all the combinations, which drives me nuts! As i've said, the original preamps are not getting dirty, they're pushing the PI input too hard and causing that stage to distort. Which is a bit of a s**t for me actually, because it's going to compromise the effects loop i'm building into mine.

Anyway, let us know how you get on if you try that....it's a quick way to get results and it might get you just where you want to be.

I am having a look at the E80F valve, which is also known as a 6084, and it looks like it's a plug-in replacement? Have you tried it? I dunno if anyone would be making them, but i will look and see what the availability might be like. I'd be interested to hear one in action.

neil.

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PostPosted: Wed Jul 11, 2007 5:58 am 
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Ah..i forgot....

there's another trick i use quite often....goes like this.....

You know how a type of pot is always specified, with the value in Ohms and also the taper (A,B or Audio, Log, etc)? Welll...try different pot tapers. Keep the value the same but try different tapers. The specified tapers are not always the most workable in my experience.

What this does is keeps the 2 extreme settings the same....(i.e. OFF at one end and whatever is the maximum at the other extreme) but it changes the WAY it dials from one to the other. I find usually changing between A and B tapers (A is audio taper, B is Log taper usually) is enough, although there are C, D, and F tapers and other custom ones out there. I have a box full of old ones, and some experimenting usually finds a type that makes an improvement.

A couple of examples......I had some Musicman 100 watt model amp on the bench, and once it was serviced, i noticed one channel sounded really weak. I cranked it and at full volume it was fine, but all the volume didn't appear until the last 1/4 turn of the knob. I can't remember what taper pot was in there but it was either A or B. I changed the pot to the other taper and the problem was solved (went from A to B, or B to A, i can't remember). But it workked perfectly.

The Fender Stratocaster guitar....it has always been common knowledge to use 250K 'A' taper pots with the regular single coil pickups. But I have always found that nothing much happens on the volume control sweep form 0 to about 7 or 7 1/2, then there's a big hump where most of the action happens between 7 1/2 and 10. So....i change to a pot of 250K B and get a smooth volume from 0-10, much more predicatable and useable.

When i built my first mic pre-amps I used 100k A pots for the gain and volumes, as the A taper is the standard in audio circuits. One of the first things i found when i got the unit going was that the sweep from Off to Maximum was not smooth. I quickly substituted a B taper pot of the same value and got exactly the smooth wind-up i wanted. Since then i have built several and always use the B taper and it is always perfect.

So my point is....experimenting with pot tapers may bring the pot's rotary sweep into a more useful area....sometimes it can go further away from what you are after too, meaning the taper you started with is closest to ideal, but often there is an improvement. It's almost as if stuff is designed bt texty-book standards but sometimes no-one actually does a listening test. Or perhaps the 'lumpy' sweep of a certain taper is what people want.

The great thing about this is that you don't need to start changing things in the circuit, it can be a little tedious swapping pots, but i just do it quickly and temporarily until i know what taper is going to work, then find or buy a new one and install it properly. And it's worth it, as the volume control is usually the prime interface between the user and the device.

Don't forget...there is a reverse taper type, the C taper, and other ones out there. If a change from A to B makes things worse, it's likely that a change from A to C will do the trick. Let the ear be the finasl judge.

Daz, in your case, the different tapers will not reduce the gain...you will still have the same 'OFF' and the same 'MAX' but you may be able to get a sweep that stays more in the clean area with all the gain coming in later on the rotation. I have the feeling you want to get more clean volume, so the previous ideas may work better, bet even then, the taper swaps can play into how that all works for you. I have seen on the 'scope how my amp reaches maximum volume at only 1/3 to 1/2 volume on the dial, and from there on it can't get any louder but it breaks up more (of course that does 'sound' like it's getting louder, but the scope will show that technically it doesn't). A curious balance between what the technology says is right, and what our ears tell us is right.

bloody electronics......

( grin ..... )

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PostPosted: Wed Jul 11, 2007 7:34 am 
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this is a great read!! :D

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PostPosted: Wed Jul 11, 2007 2:10 pm 
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It is a great read, although I'm going to have to read it about a dozen times. Before I completely understand it and because I'm currently building the TC Is your amp close to these spec to reduce the microphonics of the EF86 Coco. Just curious. :D


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PostPosted: Wed Aug 22, 2007 2:19 pm 
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Hi Neil,
the voltage divider trick seems to work for me on the 12ax7 side. Gives alittle more headroom.
I returned everything on the 12ax7 preamp side back to stock.
I tried many other things to clean it up like take a gain stage out, reducing the cathode cap value on v2 etc but I find these seem to have an effect on thinning the tone and changing the feel in a unfavourable way for me.

Have to try the voltage divider trick on the ef85 side.

Definitely changing the cathode bias bypass cap from 220uf to 22uf will not make the amp so wooly and choke up at higher gain levels.
Also using only NOS ef86 is a must - current production ef86 sound terrible and do the amp no justice!

thanks for your help.


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PostPosted: Wed Aug 22, 2007 10:51 pm 
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moved to 30 watt forum

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PostPosted: Wed Sep 12, 2007 11:45 am 
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Hi

hey, glad to hear the voltage divider worked out for you...it can take some experimenting to find the right values and create the right ratio but it's woth it when you get a setup that pleases you. After that, it's a neat way to fine tune the amp's response. It seems to do less damage to the tone than messing with components around the tubes. I use it a great deal as i always seem to be grabbing 'building blocks' from different circuits and grafting them together and having to tweak levels between stages.

Daz, you mention the 220uF v's 22uF cathode bypass caps...i assume you mean the ones for the power valves? I haven't experimented with mine much yet, but my amp doesn't have the 'bright' caps across the volume pots....although i think i will need it on the EF86 channel.....so it is still a work in progress. Because my power tubes are set up as 2 independant pairs with their own standby switches in the cathodes, 1 pair has 220uF bypass cap and the other pair has 47uF, to give a bit of extra versatility. I am bogged down with other people's amp jobs and have had little time to work on my own in recent times....it startled me to see here how long it has been since i looked in here or posted. (I refrained from saying i was shocked). Getting older is weird....you blink and a week has gone by....

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PostPosted: Mon May 06, 2024 4:25 am 
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Experimenting with cathode resistor values on your EL84-based DC-30 clone is a great way to optimize the circuit's performance. Adjusting the cathode resistor can impact factors like gain, bias, and overall tone. Trying out different resistor values is a common technique used by DIY amplifier builders to tailor the sound to their preferences. You might even find fellow enthusiasts discussing similar experiments on platforms like omegle new, where you can exchange tips and ideas to further refine your amplifier build. Have fun experimenting and finding the sweet spot for your particular build!


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