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 Post subject: Questions on Build
PostPosted: Wed Sep 30, 2009 1:00 pm 
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I really want pre and post out for recording as well as FOH duty. And I have have read the exsisting thread.
As such I have a couple of questions?
I am planning on using the SA-412 slave out cct for my Triwatt using the same values as original. Voltage divider off the 8 ohm tap consisting of a 10k and 100. I seem to remember we modded this by adding a cap in parallel with the 100 to roll off some highs.
But I also want a pre out and want to know if it would load the cct to much if I picked my signal from either pin 7 of V2 or junction C4 VR3. Or add a resistot in series with VR3 to make a voltage divder?

If I want even less gain can I sub a 12AU7 in the long tail pair?

Any suggestions on a secondary power supply design so I can run accesories like a cooling fan in the head cab, power a di box etc.

Noticed a couple of things in the manual as well:
Pg 25 gives octal filaments as pins 4 & 5
Pg 35 mid page attach the 1K5 filter , should it read 1K 5W?
Schematic no grnd on junction C20 R39

Built a particle board test mule 24" square open back 2x12 using WGS speakers, 1 Green Beret and 1 Reaper. Have to break them in so driving them with a peavey windsor studio, with no blue marvel that amp sounds totally different. Still have to build a detuned cab for the british lead.
Have to oder some baltic today.

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 Post subject:
PostPosted: Wed Sep 30, 2009 5:04 pm 
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Still building

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Where it will be used

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 Post subject:
PostPosted: Wed Sep 30, 2009 8:26 pm 
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Re: the notes you made on manual & schematic.

Yes 1K 5W not 1K5 in this case. - layout is correct
Pg 25 octal filaments should be pins 2 & 7 - layout is correct


As for the line outs, this got dropped from the design but the plan was to follow the original in this regard.

We'll have to look over a good place to insert a pre-out too.

Thanks for the pics!!

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 Post subject:
PostPosted: Sun Oct 04, 2009 11:03 pm 
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The build is done as far as I can go. Now I am just waiting for the 1K 5W resistors. Every connection, every joint has been checked. Wiring has all been checked

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Working on this to go with the triwatt

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 Post subject: Re: Questions on Build
PostPosted: Sun Oct 04, 2009 11:48 pm 
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Taz13 wrote:
I am planning on using the SA-412 slave out cct for my Triwatt using the same values as original.

I've never heard of those. What do they do?
Taz13 wrote:
If I want even less gain can I sub a 12AU7 in the long tail pair?

If you look closely, you will see the Hiwatt phase invertor is somewhat different to the usual kind of long-tail power and it is DC coupled to the cathode follower in front. So there's a lot of carefully-balanced DC voltages in that PI, and therefore I wouldn't recommend using a 12AU7 in it. Also, since this amp is essentially a Hiwatt, it does super clean tones right out of the box, without you needing to do anything extra to it. :)
Taz13 wrote:
Any suggestions on a secondary power supply design so I can run accesories like a cooling fan in the head cab, power a di box etc

The head cab is already well ventilated, and nothing gets hot enough inside to need fan cooling. Also, it's advised that you not try to add an extra power supply inside the amp as that will be likely to introduce noise, which I'm sure you don't want. :)

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 Post subject:
PostPosted: Mon Oct 05, 2009 1:36 am 
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The SA-412 was a combo, both a bass version and guitar version were made. It was a DR-504 made by Hylight and put in a combo. Used 2 1/2 12AX7, 1 12AU7 and 2 EL34 solid state rectifier. External speaker was off the 8 ohm tap and the slave out was a 10k 100 ohm voltage divider and I seem to remember doing mods by putting a cap in parallel with the 100. Actually we did mods on everything because we could.
I worked as an amp tech in Winnipeg on Corydon about a quarter century ago. Everything was a pro rush, in some cases complete with distraught owner worse then a parent with a new born, hanging over top of you while you worked. Could have been worse, coulda been the rental guy :lol:

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 Post subject:
PostPosted: Mon Oct 05, 2009 1:40 am 
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SA412 schematic can be found here

http://www.schematicheaven.com/newamps/hiwatt_sa412.pdf

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 Post subject: Re: Questions on Build
PostPosted: Mon Oct 05, 2009 10:52 am 
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Taz13 wrote:
The SA-412 was a combo, both a bass version and guitar version were made. It was a DR-504 made by Hylight and put in a combo.

Oh, you mean that SA-412, the Hiwatt combo. :idea: For some reason I'd got it in my head that you were talking about some kind of studio rack equipment... :oops: :lol:
Taz13 wrote:
Used 2 1/2 12AX7, 1 12AU7 and 2 EL34 solid state rectifier.

I'm pretty sure that it actually used a 12AT7 ECC81 in the PI, like all Hylight Hiwatt amps of that era. The PI design in the schematic you posted is *identical* to the early 70s 4-holer PI that we also use in the Triwatt, and like I said it was designed to use an ECC81. I suspect that whoever did that drawing accidentally converted the European "ECC81" type number to "12AU7" instead of "12AT7". It's an easy enough error to make. :)
Taz13 wrote:
I am planning on using the SA-412 slave out cct for my Triwatt using the same values as original. Voltage divider off the 8 ohm tap consisting of a 10k and 100. I seem to remember we modded this by adding a cap in parallel with the 100 to roll off some highs.

That should work fine. Try it without the cap first, and then different cap values till you get the tone you're looking for. Alternatively, our fellow member Tripower455 says he uses the guts of an H&K Red Box inside his amps to give a line out from the OT secondary.
Taz13 wrote:
But I also want a pre out and want to know if it would load the cct to much if I picked my signal from either pin 7 of V2 or junction C4 VR3. Or add a resistot in series with VR3 to make a voltage divder?

If you want some good Hiwatt tone in your pre out, I would recommend taking the signal from much later in the preamp. The cathode follower at V3 pin 3 (via a 0.1uF cap) may be a good place, as the signal has passed through the whole preamp at that point, and it's also at a nice low impedance. :)

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 Post subject:
PostPosted: Mon Oct 05, 2009 1:54 pm 
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Well, the 12AU7 NOS tubes I have used to live in my SA412, I know they were used. I have repaired a few and although just about every amp I worked on had all 12AX7 in v1,v2 and v3.
There are times I want real low gain and other times where an extra gain stage is desired.
For DI's I normally use Berhingers rather then redbox and am planning on building one into the cab, hence the desire to have an internal power supply.
Also planning on a guitar holder/stand on the cab sides.
But have lots of work to keep me busy until then.


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 Post subject:
PostPosted: Tue Oct 06, 2009 10:22 am 
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Taz13 wrote:
Well, the 12AU7 NOS tubes I have used to live in my SA412, I know they were used.

I believe you. :D However, it won't have come from the factory like that. All the "4-holer" Hiwatts shipped with a 12AT7 in the PI position, including the SA combos. Also the consensus among Hiwatt gurus seems to be that a 12AU7 in the PI doesn't make the amp much cleaner, mainly just quieter. They also tend to think like I do, that the 12AU7 shown in that particular SA412 schematic is an error.
Taz13 wrote:
I have repaired a few and although just about every amp I worked on had all 12AX7 in v1,v2 and v3.

A lot of rock players used to swap out the 12AT7 for a 12AX7 to get a bit more gain and crunch. Also from the late '70s onward, the PI design got changed, with a 12AX7 fitted as standard. So yes, there are a lot of Hiwatts with all 12AX7s, whether earlier models or later ones.
Taz13 wrote:
There are times I want real low gain and other times where an extra gain stage is desired.

Sounds like the Triwatt is just the amp for you then. :D The preamp version it uses is the cleanest, lowest-gain Hiwatt preamp there is. Then you've got Biacrown's Lead 100 style boost stage for when you're in the mood for more gain. The difference between the Triwatt and the Lead 100 is that we have made the boost stage switchable, whereas in the Lead 100 it was on all the time.
Taz13 wrote:
For DI's I normally use Berhingers rather then redbox and am planning on building one into the cab, hence the desire to have an internal power supply.

Yes, I like Behringers as well, and have used some of their clever boxes myself. However, you don't have much spare room inside that chassis for extra stuff, and you may also run into noise issues. Tripower455 built his Triwatt into a full size Hiwatt chassis, so he has the extra room. So personally, I would just have the two line output jacks and then run the DI equipment externally, on a separate power supply.

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 Post subject:
PostPosted: Tue Oct 06, 2009 1:30 pm 
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Resitors came in and are installed, amp passed tubeless power up. Friggin cats unknown to me were playing on the work bench. I had 12 12AX7 and now 2, had 8 6V6 now 2, broke all 6 of my KT88s :evil: :x Should have put them away after taking out what I wanted. :oops:
Lets bring back cat gut as a material for strings and I will supply the first two :twisted:

So first tube test will be with 2 12AX7, 2 12AT7, 2 6V6

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 Post subject:
PostPosted: Tue Oct 06, 2009 3:32 pm 
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Oooh, that's really bad luck. Is one of your cats black?

But you're close!!

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PostPosted: Tue Oct 06, 2009 3:43 pm 
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Bias is unadjustable and when going through the lay-out and schematic I came upon this ?
Bias problem itself was caused by grounding the Pot. I had inadsvertantly dripped solder on from the connenction to the pot housing. Thats fixed.
In one switch position I have 15 - 18 ma and the other 23 - 28 ma with 6V6 installed.
R42 on the schematic is shown to be connected in series with Junction R30 and R41 and parallel to 1/2C21 to ground.
On the layout it is shown connected in series to junction R40 and R41 and parallel 1/2C21 to ground.
I think the lay out is wrong but my theroy is a little rusty
Low distorted output from normal, bright and OD
Using 4 ohm speaker as load on 4 ohm tap
Back to troubleshooting

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 Post subject:
PostPosted: Tue Oct 06, 2009 3:48 pm 
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coco wrote:
Oooh, that's really bad luck. Is one of your cats black?

But you're close!!

Image

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Tele's make the best noise


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 Post subject:
PostPosted: Tue Oct 06, 2009 7:09 pm 
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Troubleshooting complete :D
Clean, LOUD :shock: :shock: :shock:
Bias works backwards as wired to layout
Changed R42 to conform to schematic
Robbed a 12AX7 from another amp for voltage checks
V1,2,3 all 12AX7 V4 12AT7 V5,6 6V6
Found plate voltage on V4 was 419 on each plate
Open cct somewhere and the voltage at V4 pins 3 and 8 lead me straight to it. My jumper between R25 and R36 was in fact an optical illusion. Just 1 lead that looked like it was soldered but in fact an open cct. :oops:

Biased at 19.7

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 Post subject:
PostPosted: Tue Oct 06, 2009 7:10 pm 
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Taz13 wrote:
Friggin cats unknown to me were playing on the work bench. I had 12 12AX7 and now 2, had 8 6V6 now 2, broke all 6 of my KT88s :evil: :x Should have put them away after taking out what I wanted. :oops:

That's a disaster!!! :shock: Even worse than when I threw a NOS Mullard 12AX7 out in the trash. :( I still bear the emotional scars from when that happened.
Taz13 wrote:
So first tube test will be with 2 12AX7, 2 12AT7, 2 6V6

Those should be OK to test with. Just be aware that overdriven 12AT7s don't generally sound too good, as they tend to generate a lot of high odd-order harmonics, particularly 9th and 11th.
Taz13 wrote:
In one switch position I have 15 - 18 ma and the other 23 - 28 ma with 6V6 installed.

The 15-18mA position is for the KT66s. The current seems a little high in the 6V6 position. It should hit around 20mA-ish at mid travel of the pot.

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 Post subject:
PostPosted: Tue Oct 06, 2009 9:06 pm 
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Conngratrs on the build and BTW, thanks for the feedback on the manual, schematic & layout. Checking now.

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 Post subject:
PostPosted: Wed Oct 07, 2009 1:29 am 
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Even worse than when I threw a NOS Mullard 12AX7 out in the trash.
I am in my 50s and several of the 12AX7 and 1 pair of KT88s were older then I am. I scrounged them when I was a rad tech 221 in the can. forces. They taught us tubes and fault finding to component level, so I saved components. As someone wrote elsewhere on this forum "I don't know what I have" when it comes to components, guitar hardware, etc. I have been dis-assembling things for years, DIY rules.

Now that I have the amp working, the tinkering starts. Amp has a mild hum, not objectionable, but want to see if I can lower the level.

The following is with a pair of matched Tung-Sol 6V6GT biased at 19.7ma, Ruby 12AT7,V1 sovtek, V2 JJ, V3 EH

I have a pair of KT66 to try next 8)

For Bass the normal channel is what I use, no OD, everything else at 12ish, MV to taste.

For Guitar link Normal 10 Bright 9 OD 11(when engaged) Bass 1 Treble 1 Mid 11 Pres 12 MV anywhere and while I read somewhere about no ear bleeding volumes, this comes close :shock: :D

OD not engaged and a Tokai made Ricky 360-12 copy sounds sweet and smooth so clean and clear that you can hear every mistake perfectly :oops: :roll:

I find the OD to have a little bit of a buzzy background noise probably the extra gain or tube. Hum doesn't change when OD engaged. Looks like I am going to want the footswitch option.

I hate that progressive power switch and admit it is mostly a learning curve thing.

Still have to build head and speaker cabs, waiting for BB plywood to come in. Rough Orange tolex for the head, Green for one cab and smooth blonde for the other.

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 Post subject:
PostPosted: Wed Oct 07, 2009 11:17 am 
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Taz13 wrote:
Troubleshooting complete :D
Clean, LOUD :shock: :shock: :shock:

Congratulations!!! :D

Somehow, I missed that post of yours there.

Taz13 wrote:
Bias works backwards as wired to layout.

This one has been discussed before. IMO it depends on one's definition of forward and backward. :) The problem is that when you increase bias voltage, the current drops, and vice-versa. So it depends whether you're looking at "forward" and "backward" in terms of voltage or of current. :)

Taz13 wrote:
For Guitar link Normal 10 Bright 9 OD 11(when engaged) Bass 1 Treble 1 Mid 11 Pres 12 MV anywhere and while I read somewhere about no ear bleeding volumes, this comes close :shock: :D

With the MV turned fairly low, you can get the crunch from the OD, without any need for ears bleeding. :D

Taz13 wrote:
I find the OD to have a little bit of a buzzy background noise probably the extra gain or tube. Hum doesn't change when OD engaged.

There's a lot of gain in the amp when the OD's engaged, so it will pick up any buzz floating around. It could even be from the lights in your workshop. For the hum try pulling out the 12AX7s one by one, starting with V1, and see when it disappears.

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PostPosted: Wed Oct 07, 2009 5:08 pm 
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Quote:
This one has been discussed before. IMO it depends on one's definition of forward and backward. The problem is that when you increase bias voltage, the current drops, and vice-versa. So it depends whether you're looking at "forward" and "backward" in terms of voltage or of current.


No the problem is the manual on page 48 says to turn bias control down all the way anti-clockwise. Doesn't bother me as long as I can get the bias set.

I can work with conductivity, current, voltage, phase and frequency in either conventional current flow or Electron theory :wink:

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