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 Post subject: Plexi with 6CM6s
PostPosted: Tue Sep 11, 2012 10:24 pm 
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Here are a couple of new clips. My amp now has the 1987 spec in the preamp. PI has 470R/22K. First clip is with El84s and the second with 6CM6s. The guitar and amp settings were identical. The only difference is I had to put the 5.6K grid stops in for the 6CM6s. Both clips the amp was biased at or near 70% (I think the 6CM6 was at 65%).

bright channel
Gain: 10
Master: 10
T: 7
M: 3
B: 8
P: 8
PPIMV: 10 (Kickin the snot out of the EL84s with 22K PI tail)

I also added a touch of post EQ and reverb.

EL84
http://soundclick.com/share.cfm?id=11882169

6CM6
http://soundclick.com/share.cfm?id=11885813

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Wed Sep 12, 2012 12:13 am 
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Here are some shots of the 6CM6s. They are not much bigger than the 12AX7s.
Attachment:
6CM6_1.jpg
6CM6_1.jpg [ 100.8 KiB | Viewed 7994 times ]
Attachment:
6CM6_2.jpg
6CM6_2.jpg [ 73.88 KiB | Viewed 7994 times ]


And after Jamming for a bit, I suddenly realized what I was drinking...
Attachment:
BrownAle.jpg
BrownAle.jpg [ 109.41 KiB | Viewed 7994 times ]

Nothing like a little "Brown Ale" to go with your "Brown Sound". :lol: 8)

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Wed Sep 12, 2012 8:18 am 
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Wow I really like the tone of the 6CM6 compared to the EL84s. They just seem much fuller and more vibrant.
What were your thoughts on the two tubes? What are you using now?

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Wed Sep 12, 2012 8:58 am 
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Newcie Brown - Right on!!

I flipped back & forth and agree. The 6CM6 have more meat & bottom end. The 84's seem thinner and less full. I imagine other EL84 tubes will sound different, but in this case, I likes the 6CM6 better.

Was the volume level about the same? Breakup at the same or close levels etc.

It would be interesting to hear your comparison. Tone, Feel, breakup etc.

So the ONLY change you made to run the 6CM6 was the grid stopper resister?

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Wed Sep 12, 2012 9:10 am 
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:idea: I can see an "m6" 18 watt model (switchable power tubes from EL84 to 6CM6) similar to our v6 model.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Wed Sep 12, 2012 12:47 pm 
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I'm not understanding the benefit of an "m6" over a "v6" in terms of a build option. I understand the benefit in terms of a retrofit....I've done this mod to my own 18 watt (like it a LOT) because I didn't want to punch the socket holes out for octal sockets & I wasn't concerned about the lesser pwr handling of the 6cm6's.....but you're much more limited in available 6cm6 choices. They ARE cheap, tho. I have a handful of NOS RCA 6cm6 tubes I got pretty cheap a while back. Tube Depot & The Tube Store sell "Various brands" and "Assorted brands" for next to nothing, but prices for specific NOS flavors on 3bay seem to be drifting higher.
It WOULD be interesting to be able to A-B 6cm6's and 6v6's & see what the tonal differences might be.....I'm MUCH too lazy to set that up myself, tho.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Wed Sep 12, 2012 10:15 pm 
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kurtlives wrote:
Wow I really like the tone of the 6CM6 compared to the EL84s. They just seem much fuller and more vibrant.
What were your thoughts on the two tubes? What are you using now?

The M6s seem to have a little more bottom end thickness and a little less top end. The 84s seem to be smoother and brighter.

So far I am liking the M6s. I only have about 2 and a half hours on them and haven't really had a chance to crank em loud yet but so far they sound and feel good.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Wed Sep 12, 2012 11:51 pm 
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coco wrote:
Newcie Brown - Right on!!

I flipped back & forth and agree. The 6CM6 have more meat & bottom end. The 84's seem thinner and less full. I imagine other EL84 tubes will sound different, but in this case, I likes the 6CM6 better
Yeah the M6s do seem thicker. I have 2 different sets of 84s and they both sound thinner and brighter than the M6s.

coco wrote:
Was the volume level about the same? Breakup at the same or close levels etc.

It would be interesting to hear your comparison. Tone, Feel, breakup etc.
Volume levels were about the same. I know the 84s were definitely breaking up because I could hear them. But with the M6s, I do not hear much difference in breakup with my PPIMV at 5 vs 10. With the 84s I hear a difference as the tubes get more saturation as I turn the PPIMV up. You can definitely hear it in the clip of the 84s. You can hear what some call a "smearing" of the tone.

As far as tone and feel go, the tone is a little darker. As I said earlier the M6s seem to have a little more bottom end and a little less top end. Not as bright as the 84s. Single coils sound better with the M6s. With the 84s I had to turn the treble down to around 4 or 5.

Feel is a little tighter/stiffer. It may be because I'm not driving the M6s as hard (still may need some tuning around the PI and bias). With the 84s when I would dig in the notes would get soft and squishy and compressed. Not getting that as much with the M6s.

coco wrote:
So the ONLY change you made to run the 6CM6 was the grid stopper resister?
Well I had already put in the 22K tail resistor in the PI (more on that in a minute). The only other change was to the fixed bias supply. I had to change the resistor from 10k to 33k because I was not getting enough negative voltage for the M6s. When I first turned it on and checked the bias they were running at 58 milliamps cathode current. :bugeye: After changing the resistor I got em down to 25 milliamps where they should be. Also I have my negative feedback switchable between 47k and 147k. With the 84s I was running the 47k(more feedback) to dampen the output more. With the M6s I went with the 147k(less feedback) to allow more drive.

So back to the PI tail resistor. I still have some buzz/fizz and I also noticed that even when I turn the PPIMV way down, the buzz/fizz is still there a little. So that got me thinking the problem isn't just in the power tubes. (I should probably start another thread for this.) The short of it is that I thought with the 1959/1987 preamp spec I must be slamming the PI pretty hard so I thought I would try lowering the PI tail to reduce breakup in the PI and just turn down the PPIMV to save the 84s. Now with the M6s, it seems I might even be able to go a little lower. I might try 10K in the tail a see what it sounds like.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Thu Sep 13, 2012 12:12 am 
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b0b0 wrote:
I'm not understanding the benefit of an "m6" over a "v6" in terms of a build option. I understand the benefit in terms of a retrofit....I've done this mod to my own 18 watt (like it a LOT) because I didn't want to punch the socket holes out for octal sockets & I wasn't concerned about the lesser pwr handling of the 6cm6's.....but you're much more limited in available 6cm6 choices. They ARE cheap, tho. I have a handful of NOS RCA 6cm6 tubes I got pretty cheap a while back. Tube Depot & The Tube Store sell "Various brands" and "Assorted brands" for next to nothing, but prices for specific NOS flavors on 3bay seem to be drifting higher.

I agree. I only went with the M6s because I did not have the octal holes for the V6s. One thing I read is that it is hard to find matched pairs of 6CM6s and they seem to have a varying range of transconductance even from the same brand. I found a matched pair of NOS Sylvania 6CM6s for $13. The guy had 2 more pairs that I might get just to have on hand.

b0b0 wrote:
It WOULD be interesting to be able to A-B 6cm6's and 6v6's & see what the tonal differences might be...

There's another project for you Stephen, a switchable 6CM6 and 6V6 amp for A-B ing them. Then just take out the M6s and put back the 84s when you are done. 8)

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Thu Sep 13, 2012 5:48 pm 
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More projects I dont need, but a EL84/6CM6 switchable is interesting becausewe already have enough holes to do that.

In summary, how loud does it get compared to the 18 watter? That Plexi is very loud.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Thu Sep 13, 2012 7:15 pm 
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Still not sure why 6cm6 is that attractive as a build option. Isn't a 6cm6 supposed to be, functionally, a 6v6 equivalent in a 9-pin package? You'll definitely want to massage the voltages if you're pairing them with el84's.

Here's what my datasheets say:
6cm6
315v max plate
285v max grid 2
12w max plate dissipation

6v6
350v max plate
315v max grid 2
14w max plate dissipation

The reason I'm curious how 6v6's and 6cm6's A-B is I'd like to see if there IS any tonal benefit/penalty/difference in using 6cm6's in place of 6v6's.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Thu Sep 13, 2012 8:25 pm 
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b0b0 wrote:
Here's what my datasheets say:
6cm6
315v max plate
285v max grid 2

That's what my datasheet says as well but I am exceeding that. I am at:
338v plate
334v grid2
coco wrote:
In summary, how loud does it get compared to the 18 watter? That Plexi is very loud.
Just got a chance to crank it. This thing is freaking LOUD! :shock: It is definitely louder than the EL84s. I don't know how much louder but with the 84s I could stand in front of the cab but with the M6s I could feel the pressure in my ears. I had to step out of the room. My ears are ringing after only about 15 - 20 minutes of playing. And man it sounds fantastic cranked

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Thu Sep 13, 2012 8:31 pm 
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Ok. I'm ordering some!!

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Thu Sep 13, 2012 9:16 pm 
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Here's one at around -9db to -12db I don't know you would have to shout loudly to be heard. Bright channel, everything dimed, 47k feedback for a little more dampening. It is sounding very thick and crunchy like a plexi should. 8)

http://soundclick.com/share.cfm?id=11889245

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Fri Sep 14, 2012 8:20 am 
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How close is your Plexi Brown to the schem in the layout section?
I hear you mentioning NFB and fixed bias which are pretty far from the original 18W.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Fri Sep 14, 2012 11:38 am 
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My amp is now basically a 1959/1987 spec with 18 watt power section. I have a cascade switch which makes the first 2 stages (V1A and V1B) the 2203/2204 (JCM800) spec. I have a FAT switch instead of the boost, switching between 330uf and .68uf over 820R on V2A. I have NFB switch that switches between 47K and 147K for tighter/softer feel. And I have added a PPIMV. So yes, it is quite a bit different. The values highlighted in red on the Plexi Brown schematic are basiclly the values from 1987 spec added to the 18 watt spec plus a few brown values. That is how I first built my amp over a year ago. There are also a few (but not all) gain reducing changes as I found that the 1987 spec values produce a bit too much gain for the standard 18 watt PI and EL84s. Now with the 6CM6s, I have removed all the gain reducing values (and brown values for that matter) and have gone to just the stock 1987 spec.

The NFB loop really helps dampen and reduce gain in the output but that really changes away from the 18 watt design. I do not notice much difference between fixed vs cathode bias but since I added it, I just always use fixed bias. You can do without the NFB and fixed bias but when using EL84s I think you have to have the gain reducing values and the PPIMV. VVR/VRM is just not the same. You need to reduce the input to the EL84s. I used to run mine around 8 and it sounded good. Also I found that 470R bias resistor in the PI was too hot. I went back to 820R and that helped as well.

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Mon Sep 17, 2012 6:57 pm 
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I really dig those clips of the 6cm6's! My amp has the stock TMB, cathode bias circuit. What would I need to do in order to run the 6cm6 in my amp? Is it just a resistor value change?


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 Post subject: Re: Plexi with 6CM6s
PostPosted: Mon Sep 17, 2012 9:15 pm 
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You would have to change the cathode resistor and cap. I think Zaphod said 300R/330uf. Maybe someone can chime in as to how to calculate the right value. You will need to change the PI tail from 56K to 22K. And .... you need to rewire the tube sockets as follows (heaters are the same):

Code:
EL84            6CM6
pin 2   G1      pin 3
pin 9   G2      pin 1
pin 3   G3K     pin 7
pin 7   Plate   pin 9

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Mon Sep 17, 2012 10:54 pm 
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For those with a Trinity Amps head, there are 2 extra holes for sockets (intended for a 6V6 tubes set). You can wire two more noval sockets, wire them up, add a second 56K tail resistor on a switch, add an extra cathode pair and , voila, switchable output stage, a variant of the v6.
That is if you want to try first!

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 Post subject: Re: Plexi with 6CM6s
PostPosted: Tue Sep 18, 2012 11:57 am 
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coco wrote:
Ok. I'm ordering some!!

Are these being made again or are the suppliers relying on NOS.


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