Converting 50W Plexi to 20W 6V6

[shocki]

There was a similar thread some time ago which I cannot find right now. I build a 50W plexi that sounds really great. But I would like to test if its possible to bring the volume (clean headroom) down. (I know there might be other possibilites like MV which could make more sense)

My PT has 325V which is OK. When I run my 8 Ohm speaker with the 16 Ohm OT setting the tubes should see 6400 Ohms which is also OK for 6v6. I have to change a little thing about the bias and the grid resistors as described here.
http://mhuss.com/Plexi6V6/Plexi6V6c.gif

Very minor changes actually.

Now to the question If the OT stays the same (50W) will this give me the same sound as if I would switch to a 20W OT (everything else being the same). In the thread back than someone said it would not matter cause the distortion comes from the tubes anyway. Probably the bass will be a bit more tight with the bigger iron. But other than that would the amp have around 20W when changing to 6V6?

[big_teee]

There was a similar thread some time ago which I cannot find right now. I build a 50W plexi that sounds really great. But I would like to test if its possible to bring the volume (clean headroom) down. (I know there might be other possibilites like MV which could make more sense)

My PT has 325V which is OK. When I run my 8 Ohm speaker with the 4 Ohm OT setting the tubes should see 6400 Ohms which is also OK for 6v6. I have to change a little thing about the bias and the grid resistors as described here.
http://mhuss.com/Plexi6V6/Plexi6V6c.gif

Very minor changes actually.

Now to the question If the OT stays the same (50W) will this give me the same sound as if I would switch to a 20W OT (everything else being the same). In the thread back than someone said it would not matter cause the distortion comes from the tubes anyway. Probably the bass will be a bit more tight with the bigger iron. But other than that would the amp have around 20W when changing to 6V6?

I don't have any answers, I too want to try the JJ 6V6s in my 2204 Clone with double MVs.
I was going to try it as is.
My Plate Voltage is 492V at a idle load.
Was planning on using the JJs because they are supposed to be good to 500V.
I will keep checking this thread and see how your experience goes.
I don't have any tubes yet.
T

[shocki]

found something here... Transformer ratings & how to interpret them

[MWJB]

@ Shocki - The bigger OT won't allow you to make more power than the 6V6 are capable of, the amp will make ~22-25W, just with slightly higher headroom & better dynamics than typical 20-25W spec'd parts.

@ Big Tee - It would be a good call to correct output impedances so that the JJ6V6s see at least 4.5K-5K primary Z. Otherwise you seem good to go (assuming a rebias).

[Gingertube]

FWIW - Just my interpretation/understanding.

The 6K8 reflected impedance from running a 16 Ohm box on the 8 Ohm speaker tap will indeed suit 6V6. I have run a quad of 6V6 into a standard 50W Marshall output transformer (Raa = 3K4) in a amp I built for a friend so that is effectively the same thing.
NOTE: This is the opposite to what you wrote above. If you want to reflect a higher impedance to the primary then you want a higher impedance on the speaker output. So you would want to connect your 8 Ohm box to a 4 Ohm output.

When you do this 16 Ohm speaker on the 8 Ohm tap thing (or 8 on 4) what is different from using a "real" 6K8 Raa transformer is that the Raa = 3K4 unit will have less primary turns and therefore a lower primary inductance. That would normally mean you lose some bass.
In this case though there are 2 offsetting considerations. 1st the tranny is oversize for the "20" Watts you will be delivering through it and 2nd guitar is midrange instrument anyway with little bass (what HiFi guys would call bass anyway).
At the other end of the audio spectrum the reduced primary turns may also give you reduced interwinding capacitance and leakage inductance so you may actually get some top end improvement.

So I strongly doubt you will notice any real frequency response change but if you do it would more likely be a better top end than a loss of bottom end.

Note I put the 20 in quotes - somehow the DIY fraternity has taken up the marketing bunko from various manufacturers - Standard max power (without way over rating the tubes) is 17W for EL84 and 14W for 6V6. In guitar amps we do often overrate the tube a little but even so very few of the "20" Watt Amps actually put out 20W RMS. That doesn't really matter much since to get double the volume of a 10 Watt Amp you need 100W and not 20W but it "offends my sensibilities" to see this B.S. so widely disseminated and then just tolerated by those who know better.

I measured the max power of the quad of 6V6 amp at 32W RMS at just below the onset of clipping. That was with 350V B+.

Cheers,
Ian

[bob p]

But other than that would the amp have around 20W when changing to 6V6?

I'm not sure that I follow your line of thought. What parameters are you using to define the target power output at 20W?

[Leo_Gnardo]

What Gingertube said.

Plus, why beat a pair of 6V6, even JJ's (cheap & tough) with nearly 500V.

How about triode mode for your EL34's? That'll get you down to 20W or so, mellow the high end too. And you could put in a switch so you can choose pentode or triode mode depending on what you need at the moment.

With a couple minutes work you could audition your amp in triode & find out if you like it. And if you don't, get back on the 6V6 trail & forget about this suggestion.

[shocki]

I'm not sure that I follow your line of thought. What parameters are you using to define the target power output at 20W?

I build this 50W Marshall which sounds really great. But I am selling this to a friend. So I want to build a second one which does not really has to be 50W for my purposes. So I looked at the 18W 6v6 Plexi Project. I just wrote 20W as a rough guide. Probably the JJ 6V6 will output about 25W. But if its 20,22 or 25W does not really matter. As long as it is a lot less than 50W.

Plus, why beat a pair of 6V6, even JJ's (cheap & tough) with nearly 500V.

My plate voltage is about 480V. Deluxe Reverbs also have very high Plate Voltages. But I might use a 300V PT in my actual build. Not sure yet. Both would be OK I guess.

With a couple minutes work you could audition your amp in triode & find out if you like it. And if you don't, get back on the 6V6 trail & forget about this suggestion.

Yepp I did this. Sounds OK.

I am not going to convert this amp (maybe I should have said that) finally to 6v6. It sounds good as it is. But I was thinking if the 6V6 Plexi is almost the same I just could try how I like 6V6 in that 50W amp and if it is gonna give me a lot lower output.
If I like I would build a second amp with a smaller OT and maybe also slightly weaker PT.

I don't have any answers, I too want to try the JJ 6V6s in my 2204 Clone with double MVs.

You would have to change the 200K bias grid resistors to 100K. At least this is what I read in the JJ6v6 Datasheet. Not sure what happens if you don't do this

[Leo_Gnardo]

I am not going to convert this amp (maybe I should have said that) finally to 6v6. It sounds good as it is. But I was thinking if the 6V6 Plexi is almost the same I just could try how I like 6V6 in that 50W amp and if it is gonna give me a lot lower output.
If I like I would build a second amp with a smaller OT and maybe also slightly weaker PT.

Good plan. I've thought about building a similar amp, JTM45 with 6V6 or EL84. With a B+ around 350V. One of these days...

[big_teee]

You would have to change the 200K bias grid resistors to 100K. At least this is what I read in the JJ6v6 Datasheet. Not sure what happens if you don't do this

The 220K Grid resistors have been replaced with the 250k Dual PPI-Master Vol. Pot.
It is variable, so don't know if that would work or not.
It works with 6L6s, EL34s, and KT77s, was wanting to try the 6V6s.
Also my Speaker array is 16 ohms.
I think I would need to put the amp on 16 ohm, into a 8 ohm speaker.
Not sure if that is 16 into 8, or 8 into 16?
T

[shocki]

Here you find a schematic with the MV.
http://i671.photobucket.com/albums/v...0/50wBuild.jpg

If the vol pot half open we have 125K and 2M parallel and 125K in series. This is 117k + 125K = 240kOhm wich is the resistance the grid sees.
If you would take a 100K pot you would have 50K and 2M parallel and 50k in series which would give you 48K + 50K = 98K.

So I think you could use a 100K Pot instead of 220K and get there for the 6V6.
Hope this correct

[big_teee]

Here you find a schematic with the MV.
http://i671.photobucket.com/albums/v...0/50wBuild.jpg

I don't really get this though. If the MV is fully open aren't the grid bias resistors 2M? And if fully closed 250K with 2M in parallel? I am thinking about the resistance from the grid to the bias supply. But maybe this is wrong?

The resistance the grid see coming from the PI is 250K which is close to the 220K the grids normally see.

I don't thoroughly understand it either.
It works great, and I wouldn't want a 50 watt or larger amp without it.
It makes the amp much more versatile.
Really with the 2 MVs the 6V6s are not needed, just wanted to try them.
I think the 2meg resistors are not required, but are a safety measure, in case the Pots go open, or don't work.
T

[shocki]

I corrected what I wrote before and added some calculations. The 2M are for saftey reasons. Yes.

Check the datasheet
http://www.tube-town.net/info/datenb...s/jj/jj6v6.pdf

Even if you don't understand it you can check there what values to use. Grid Resistance limit for fixed bias is 100K.

[big_teee]

I corrected what I wrote before and added some calculations. The 2M are for saftey reasons. Yes.

Check the datasheet
http://www.tube-town.net/info/datenb...s/jj/jj6v6.pdf

Even if you don't understand it you can check there what values to use. Grid Resistance limit for fixed bias is 100K.

The old 6V6/EL34 Trainwreck circuit used 220K Resistors.
I think my amp will work as is, just keep it biased at a low level.
Back to the OT impedance.
My OT is 3.6k.
So to double the impedance, set the amp to 16ohm, then run it into a 8 ohm speaker.
Or do you set the amp on 8 ohms and use a 16 ohm speaker?
I have trouble keeping that straight.
Hopefully I haven't hijacked your thread to badly!
T

[MWJB]

The old 6V6/EL34 Trainwreck circuit used 220K Resistors.
I think my amp will work as is, just keep it biased at a low level.
Back to the OT impedance.
My OT is 3.6k.
So to double the impedance, set the amp to 16ohm, then run it into a 8 ohm speaker.
Or do you set the amp on 8 ohms and use a 16 ohm speaker?
I have trouble keeping that straight.
Hopefully I haven't hijacked your thread to badly!
T

The 250K grid resistors/PPIMV will work fine with 6V6s, use the 8ohm tap with a 16ohm cabinet.