6L6 to 6V6 opinions

[megawhat]

Hello...

I've recently got a hot rodded JCM800 preamp working into a 6V6 power stage with some NFB coming back for the usual pres and depth...

It sounds good!

15 watt OT 8K Primary,
around 320vdc on the plates.

I'm posting because I'd like to get some opinions...
the first issue, is that I still have a big 10K/2W dropping resistor between B+2 (screens) and B+3 (PI and rest of preamp). I need to get a more detailed notes of all the voltages but I'm debating dropping that resistor down to something that will bring the preamp voltages right up...

can anyone see how I could run into a problem doing that?


next... my main issue.
The preamp was designed to run with 6L6/5881's, so by using 6V6's I know I have decreased headroom. The amp currently seems louder than I'd expect 2 6V6's in push pull to be. The circuit is SLO based and I'm pretty sure I need to decrease the level of signal hitting the 6V6's to restore headroom, and allow the amp to get louder before 6V6 breakup.

I don't currently have access to a signal generator or scope, so I was wondering if anyone had any opinion on this. I prewired the board so I could split the anode resistors on the PI, keeping the preamp character as original as possible, and just attenuating the signal out the PI

do my ideas sound fairly reasonable?
cheers

[John_P_WI]

I'm a little confused, did you build a hot rod JCM 800 or a SLO pre amp? Regardless, one of the easiest things to try is to pad the master volume before the PI with a large resistor. Tack a 1 meg across the outer legs of the pot as an experiment.

You may also need to up the grid stopper value on the 6v6 a little and can try playing with the neg FB a little too.

In my experience 2 6v6 tubes can kind of smear the tone in high gain amps, 4 6v6 tubes really crank.

Good luck,

John

[megawhat]

Its very similar to the SLO lead preamp, which is a hot rod 800 (in a way), very similar... but not like the Avenger preamp. It doesn't have high and low gain inputs. But it's pretty much the same.

I have 5K6 grid stops on the 6V6's and 1K on the screens.
NFB is via 39K resistor from the 4ohm tap.

It's sounds good so far, just the volume jumps up pretty high when I touch the MV knob. I've only been able to work on it late a night so testing it at volume has been a bit hard.

Thanks for the tip on the MV. I know the Soldano Atomic 16 has a potential divider just bofore the MV and the Decatone uses 1M's in series with the crunch and lead channel master volumes. I should have remembered that.

[iknowjohnny]

On a side note, concerning the master jump you mentioned, try a linear pot. I had the same issue and tried a linear 1M where the audio taper 1M as and it's now transitions from low to high volume much better better. I never did that before because i figured since ever master i've ever seen is audio there must be a reason why. But no, linear works a lot better.

[megawhat]

Interesting... I'd have thought you get a big jump in volume using a linear. Maybe the jump is right at the top of the sweep though and you don't get up to that volume range usually? or is it smooth all the way up?

[John_P_WI]

Another thing I forgot to mention, changing your preamp voltages, has a change in tone. You can simply parallel (temporary tack) another resistor across the one you are talking about to lower the resistance and increase the preamp voltages as an experiment.

[dynaman]

The Avenger and OD channel of the SLO are extremely similar. The Atomic 16 has one less gain stage and is closer to the 2203/4 JCM800. I've pushed 6V6s with both preamps and have had success strapping that 150K across the usual 1MA MV pot. You can try any value you like. I've even just stuck a 100KA pot in for the MV and got rid of the resistor. Worked fine with negligible loss. I don't know about using a linear pot in that position.

[megawhat]

Cheers guys,

Yeah I'd try a few values across the 10K do experiment with voltages. I know there would be a tone change but currently the voltages are lower than you'd see in an SLO or Avenger. Not that it sounds bad, but I'd like to see what happens for myself. (I've read about the B+ voltage differences effect on tone/feel)

My preamp is basically the Avenger preamp. I have no loop. I have the spare triode being used as a parallel triode up front. I halved the plate and cathode resistors on V1.

I know the atomic 16 has less gain and the potenial divider, I guess because it's driving EL84's which break up before 6V6's do even? It's nice to know I've found someone who has made this circuit. I'll try the 150K.

By dropping that level into the PI, just how much character would I be 'loosing' from the LTP?

[dynaman]

I didn't notice much except for a bit more headroom. You can also try fiddling with the values of the PI parts. I think I only did that on one build and I don't even remember what I did. I usually just keep preamp/PI plate voltages close to Soldano specs by altering that resistor in the DC supply after the screens (as you mentioned). I've never ran my 6V6s at the low voltages you have.

You can also try swapping in some lower gain preamp tubes.

[dynaman]

I'm a little confused, did you build a hot rod JCM 800 or a SLO pre amp? Regardless, one of the easiest things to try is to pad the master volume before the PI with a large resistor. Tack a 1 meg across the outer legs of the pot as an experiment.

You may also need to up the grid stopper value on the 6v6 a little and can try playing with the neg FB a little too.

In my experience 2 6v6 tubes can kind of smear the tone in high gain amps, 4 6v6 tubes really crank.

Good luck,

John

Yeah, try messing with the NFB too. John's right about the 6V6 quad. I did an Atomic 32(?) with a quad and it ripped. It's buyer ditched his stock Marshalls for that little monster.

[megawhat]

My plan was to keep the preamp voltages up like the SLO. I used parts from a local builder though so I didn't get a PT that had a lot of voltage on the HT wind. The PT is a designed to be used in a lower gain 6V6 amp.

I kept the preamp similar apart from the parallel triode (i used that as I read it's meant to reduce noise by 3dB).

That said, with the HT fuse removed, I did get 380vdc at B+1. Obviously thats with none of the rest of the supply circuit connected, so I imagine it's much lower when powered up and working.

I have read a blog where a fella made the SLO with EL91's in the power section using the split PI anode resistors. He used 25K+75K in series on one side and 20K + 60K on the other... with the coupling caps to the 6V6's tapped between the pairs. I guess the 25K & 20K resistors are on the 12AX7 side, while the 60K and 75K would be on the B+ side. quite a lot of attenuation... but then he is using EL91's.

[dynaman]

What's yer unloaded HV winding? Using a tube rectifier?

[megawhat]

What's yer unloaded HV winding? Using a tube rectifier?

SS discrete bridge, since the PT is a toroid with no CT.
I wasn't given a spec but told that after rectification I should get ~350vdc

Measuring at the first cap I got 380vdc (is a situation where the rest of the supply is cut off, like when using a standby switch just after the first filter cap).

I have a potential divider for heater ref after the 10K dropping res.
The voltage on that is 30vdc. the divider is made up of a 820K and a 100K resistor. so when fully loaded, the voltage going to the PI anode resistors must be about 280volts or just under...


sorry for not being accurate. I'm in work and all my notes (and the amp) are at home.

[megawhat]

Hello Again!

I got some voltages for digestion... It looks like I would be able to decrease that 10K/2W after B+2. I would like the amp to sound tighter. The SLO has 380 for the B+ feeding the PI I think. Obviously I won't get that, but if I can get B+3 up 50v or more (and that filters down) I should be able to hear/feel the difference right?

I'm pleased with the results so far. As long as it is stable when I get to play it at volume etc than I'll be really pleased! this is my second build!


Heater DC Ref. 30vdc
my bias range is -34 to -24vdc. I need to increase that. My 6V6's idle at a 23mA

365v = B+1 Plates
365v = B+2 Screen
290v = B+3 PI + CF stages
275v = B+4 2nd + Cold stage
270v = B+5 Input (parallel triode)

v1 anode 133v
v1 cathode 1.13v

v2b anode 193v
v2b cathode 1.47v

v2a anode 266v
v2a cathode 2.86v

v3b anode 193v
v3b cathode 1.70v

v3a anode 288v
v3a cahode 193v

v4b anode 192v
v4b cathode 32.1v

v4a anode 196v
v4a cathode 32.1v

[megawhat]

This amp is working nicely now!
I just to decide the best way to reduce some level into the 6V6's.

Some ideas I've seen but I'm not entirely sure on the best method...
Potential Divider before the MV (im using a 1M log pot for MV pre PI)
A resistor (maybe 150K) across the outer lugs on the MV)
Splitting the PI anodes to feed the 6V6's (I am using NFB injected into the PI tail).

any opinions on those ideas?