6V6 plexi (with 18 watt preamp) question

[dude]

Hi all and Happy New Year to bunch of great guys,

I gutted a Crate VC20, and put in a PTP board making my own schematic from a 6V6 plexi and an 18 watt lite front end.

I used the PT that was in the VC20 (small cheap PT only putting out 318v B+ with SS diodes), used a bigger OT (about 18 watts) then the Crate's original as that was small and 16 ohms.

The circuit just doesn't have the volume as some other 18 watt amps and is missing that plexi 6V6 cutting tone, lacking some high end to cut through. Also the presence control with a 27K doesn't seen to have a lot of effect on the tone (had a 100K and 33K). Hard to see the the wiring on the schematic but I'm sure I have it right on the presence.

Any suggestions on getting closer to say Slucky's 6V6 or Mark Huss's. I know the front end is an 18 watt and I have only one volume and gain pot, don't expect the exact same tone as a real 6V6 plexi. Maybe getting rid of the gain pot and have two volumes but not sure how to do that?

The amp sounds ok, no problems, just what I mentioned above. I wanted to use the same panel as the VC had, so I made the schematic to match the panel pots.

I was thinking of putting in another PT that would raise the voltage to around 360/375v, (290-0-290) would that help?  I biased the 6V6 @ around 20 mA. 

This is my first circuit I put together.

Thanks in advance,
al

[ac427v]

Hey Dude, it works! Always a good outcome with a new circuit. I keep waiting for my son's VC-50 to die so I can do something like this.
Before going to the work and expense of a new power transformer, you could try to get more working voltage out of this one. I don't know the math or theory, so I have to trial and error it by reducing the size of the resistors in the power supply dropping string by about half. 1k-3 watt to screens, 4.7k-1 watt to phase inverter supply, and 10k-1 watt to preamp supply. Then rebias the power tubes to 26-28 ma and see how it sounds.
Craig

[dude]

There's no way to increase the B+ that I know of, only decrease down the power rail. I have 235v at the plates of the first stage, that's high enough to give a loud clean tone, IMO.

I just thought someone might have a suggestion and I wanted to know if increasing the B+ to plates and screens of the 6V6s, might have a effect on output and put more treble to cut threw. It's kind of Brown Tone sounding now, I'm assuming it's the low voltage to the screens of the 6V6s...?

Maybe some one will eventually lead me in the right direction. Maybe I'll try increasing the fixed bias but I'm at about 80% dissipation now.

Thanks for your answer,
al

[shooter]

doesn't have the volume

might try just gator clipping a ||(parallel) 470k with the one coming off your gain/vol pot wiper feeding V1B.  you might not have enough drive outta the pre.?

[ac427v]

Hi Al,
I didn't mean to imply that you can increase B+. I say make the most of what you got. That means increasing voltage in other parts of the amp. That "brown sound" often results from low preamp voltage. I don't think you have 235v on the preamp plates. You show 235v supply to the preamp plate resistors. What do you measure on the plate side of those resistors? You want at least 200v on the plates for loud clean tone in an amp that has only one 12AX7 preamp tube and a full tone stack to drain off signal. Smaller dropping resistors in the power supply string will boost plate voltages on the tubes downstream. That approach would be much easier and cheaper than a new power transformer.

I like to bias 6V6 tubes much hotter when the supply voltage is only 318v. You may be comparing to a deluxe reverb with a 420v supply biased at 19-20 ma. Your amp is different. It runs on 100 volts less supply and needs higher bias to work those power tubes.
Happy New Year!!!
Craig

[dude]

doesn't have the volume

might try just gator clipping a ||(parallel) 470k with the one coming off your gain/vol pot wiper feeding V1B.  you might not have enough drive outta the pre.?

So, cutting that R to 235K would increase gain? Think a higher B+ would help too?

I'll have to open up the amp, AC427, I forgot if that was the rail voltage or the voltage after the plate resistor (100K) to V1A, if it was before that still would be around 180 /200v at the first stage plate. I think Brown tone voltage there is around 140/150v...?

alr

[DummyLoad]

have you tried fiddling with the NFB loop? seems like those values would be OK with bassman taken off a 2 ohm tap. for 8 ohm tap try experimenting with 56K-82K for NFB resistor. another method to find the value of the NFB R is to replace your 27 resistor with a  250K pot and find the sweet spot were the "presence" control just starts having an effect, then measure pot and install the closest standard value.


with 8K Z load OT and 300V you should be making about 15W. that's fairly loud for a guitar amp. what speaker are you driving? have you tried others?


--pete

[dude]

Good suggestions.

Since Neg feedback pot doesn't change presence that much, the 250K pot to get the right value sounds like a good idea, I'll try that, thanks D Load. Your right I wasn't thinking about impedance of the 8 ohm speaker.  I had a 100K there and there was hardly any difference when adjusting the NF, maybe try a 100K pot if i have one, if not I'll use a 250k.

Also, the cut down the 470K to the grid of V2A off the gain as Shooter mentioned, I guess I could alligator a 500K pot there too to see the effect of different valves.

I'll check the voltage on the plate of V1A, if it isn't over 200v, I'll adjust the rail as ac427 suggested, up the bias too. I wasn't thinking about the low B+ when I set it to 20mA, I didn't do the math.

Thanks all I'll try these one at a time and post. Might take me a few days as I picked up a nasty cold.

al 

[DummyLoad]

Good suggestions.

Since Neg feedback pot doesn't change presence that much, the 250K pot to get the right value sounds like a good idea, I'll try that, thanks D Load. Your right I wasn't thinking about impedance of the 8 ohm speaker.  I had a 100K there and there was hardly any difference when adjusting the NF, maybe try a 100K pot if i have one, if not I'll use a 250k.

Also, the cut down the 470K to the grid of V2A off the gain as Shooter mentioned, I guess I could alligator a 500K pot there too to see the effect of different valves.

I'll check the voltage on the plate of V1A, if it isn't over 200v, I'll adjust the rail as ac427 suggested, up the bias too. I wasn't thinking about the low B+ when I set it to 20mA, I didn't do the math.

Thanks all I'll try these one at a time and post. Might take me a few days as I picked up a nasty cold.

al

not sure you understand what i'm trying to convey. KEEP the 5K pot as is: TEMPORARILY replace the 27K NFB resistor with a 250K pot. dial up the 250K pot to were the 5K presence pot works best. when you have it where you like it, shut down the amp and measure the 250K pot, then you select a fixed resistor that's close to what you measured on 250K pot.


--pete 

[dude]

Yeah Pete, I got it, that's what  I was going to do. I was just saying I originally had a 100K in the loop and notice not much change - going lower gave more change if I remember right. So, I was saying I'd use a 100K pot in place of the 27K r to dial in the best change, I'm sure 250K would ok too. Maybe more than 100k in the loop might have more presence due to the 8 ohm impedance speaker...?

thanks,
al

[MFowler]

Some suggestions


V1 A&B bypass caps from 1uf to 25uf.


Voltage divider 470k/470k to 220k/220k



PI 10k tail to 47k and PI bias resistor from 470R to 1k2


100k/100k power amp bias resistors to 220k/220k


Mark