Just finished putting together this 59' Supro Clone (schematic attached)
To limit cost I am using parts from a previous amp build.
PT is a Hammond 269ex 190-0-190 65mA. which differs from the 270-0-270 listed on the schematic. I am using diodes instead of tube rectifier. Also using different PSU-RC values:
1st cap: 47uf/450vdc to 470R/ 2W series resistor
2nd cap: 10uf/450vdc to 1k/1W series resistor (OT Primary)
3rd cap: 10uf/450vdc to 10K/1W series resistor (6v6 screen)
4th cap: 4.7uf/450vdc (12ax7 plates)
Voltages are:
6V6 anode = 250
6V6 screen = 235
6V6 cathode = 13
12ax7 anodes = 170 (using 100K anode resitors & 4.7k cathode resistors)
240R/1W 6V6 cathode resistor
I am getting a crossover or blocking distortion. Tried grid stoppers but didn't help. The voltages are very low for a 6v6 which are usually run on 300v and higher. So, I am assuming that the cold bias of the 6V6 is the most probable source of the bad distortion. Originally I set up the preamp as it is in the schematic, but was getting 80v on the 12ax7 plates causing loose distorted sound. Lowering the plates resistor values to 100k and cathodes to 4.7k got it to 170v and improved the sound.
I tried lowering the PSU-RC resistors to 100R/1K/10K but that raised the screen voltage higher than the plate voltage (using 240R cathode resistor)
Is there a safe way to get closer to 300 volts on the 6V6 plate. Reading taken at PSU side of diodes is 300vdc. At resevoir cap 292vdc
I layed it out as best as I could to avoid lead dress issues. Using split ground scheme.Not using NFB
59' Supro Clone 6V6, 250 Plate voltage
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59' Supro Clone 6V6, 250 Plate voltage
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martin manning
- Posts: 14308
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: 59' Supro Clone 6V6, 250 Plate voltage
First, since it's single-ended, it can't have crossover distortion, but I think your 6V6 is OK where it is. What is the OT primary impedance it's working into?
A 100k plate load with a 4k7 cathode resistor will bias the 12AX7's near cut-off. Are both pre-amp stages set up like that? Maybe try something smaller for Rk's, like 1k5's? Going to cathode bias on the first stage is a good idea, I think; you'll want to change the 5M6 grid leak resistor to a 1M if you haven't already. What's the supply voltage for the pre-amp? About 200?
A 100k plate load with a 4k7 cathode resistor will bias the 12AX7's near cut-off. Are both pre-amp stages set up like that? Maybe try something smaller for Rk's, like 1k5's? Going to cathode bias on the first stage is a good idea, I think; you'll want to change the 5M6 grid leak resistor to a 1M if you haven't already. What's the supply voltage for the pre-amp? About 200?
Re: 59' Supro Clone 6V6, 250 Plate voltage
OT primary is 5k. I thinking of trying 47k or 68k plates resistors and lowering the cathodes around 820R or 1k for the preamp,martin manning wrote:First, since it's single-ended, it can't have crossover distortion, but I think your 6V6 is OK where it is. What is the OT primary impedance it's working into?
A 100k plate load with a 4k7 cathode resistor will bias the 12AX7's near cut-off. Are both pre-amp stages set up like that? Maybe try something smaller for Rk's, like 1k5's? Going to cathode bias on the first stage is a good idea, I think; you'll want to change the 5M6 grid leak resistor to a 1M if you haven't already. What's the supply voltage for the pre-amp? About 200?
since the low power tube voltage allows for less headroom.
The input stage grid leak is 1M,and I removed the .05 cap in front of it.
Preamp B+ is 205 volts.
Re: 59' Supro Clone 6V6, 250 Plate voltage
Yes use the 1M grid leak on the first stage.
The 100K plate and 1.5K cathode resistors put the 12ax7 in the center of it's operation window. You will get the best clean tones in this area.
If you want more drive then change the plate to 150K and 2.2K cathode.
Same with the second stage.
I would also include a 1.5K grid stopper resistor on the 6V6 grid.
The 100K plate and 1.5K cathode resistors put the 12ax7 in the center of it's operation window. You will get the best clean tones in this area.
If you want more drive then change the plate to 150K and 2.2K cathode.
Same with the second stage.
I would also include a 1.5K grid stopper resistor on the 6V6 grid.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: 59' Supro Clone 6V6, 250 Plate voltage
I just lowered replace the 470R to a 100R in the filter stage and added a 1K screen resistor . Now I have 275v on the 6V6 plate and 255v screen, 12v on cathode.
I also changed the preamp plate resistors to 50k, and cathodes to 820R.
Helped a bit. Removed 6V6 220uf cathode bypass cap and it helped a bit with the distortion problem, but hurt the tone.
I'll use your suggestions, hopefully it will sound better. Might have Intermodular Distortion going on ? I hooked up 10K NFB resistor between 2nd stage top of cathode resistor and the positive output jack tip. Made it sound a bit worse, but wasn't squeeling.
I also changed the preamp plate resistors to 50k, and cathodes to 820R.
Helped a bit. Removed 6V6 220uf cathode bypass cap and it helped a bit with the distortion problem, but hurt the tone.
I'll use your suggestions, hopefully it will sound better. Might have Intermodular Distortion going on ? I hooked up 10K NFB resistor between 2nd stage top of cathode resistor and the positive output jack tip. Made it sound a bit worse, but wasn't squeeling.
Re: 59' Supro Clone 6V6, 250 Plate voltage
This looks like Supro's take on a Champ with a Vibrolux-style input (except for the first-stage grid-leak bias).
I like Structo's 100K/1K5 suggestion for the first stage. You could try 220K/2K7, or just bypass the cathode with a 25uF/25V (or 50V) cap. The Fender 5F1 schematic shows the first stage unpypassed BUT every 5F1 I've ever seen actually has the bypass cap. For the second stage, 100K/1K5 UNBYPASSED should also work.
10K feedback resistor will give an awful lot of feedback on top of a 4K7 cathode resistor. If you go with 1K5, use 22K or 33K for feedback.
I think the 6V6 will be happier with the 470R traditional value cathode resistor, but it doesn't look like you're killing it with the 240R. I think a 1W 240R will get pretty toasty tho.
I know that the modern fashion is large bypass caps on output tube cathodes, but I always preferred the sound of the old 25uF tried and true.
Good luck.
I like Structo's 100K/1K5 suggestion for the first stage. You could try 220K/2K7, or just bypass the cathode with a 25uF/25V (or 50V) cap. The Fender 5F1 schematic shows the first stage unpypassed BUT every 5F1 I've ever seen actually has the bypass cap. For the second stage, 100K/1K5 UNBYPASSED should also work.
10K feedback resistor will give an awful lot of feedback on top of a 4K7 cathode resistor. If you go with 1K5, use 22K or 33K for feedback.
I think the 6V6 will be happier with the 470R traditional value cathode resistor, but it doesn't look like you're killing it with the 240R. I think a 1W 240R will get pretty toasty tho.
I know that the modern fashion is large bypass caps on output tube cathodes, but I always preferred the sound of the old 25uF tried and true.
Good luck.
Re: 59' Supro Clone 6V6, 250 Plate voltage
Pics added. This is a P2P build, still in testing, loose parts will be shrink wrapped and or put on a terminal. Had to improvise with bridge terminals, as I am running low and don't want to pay shipping for $3 worth of parts.
As I mentioned before I am using split grounds, and tried to keep the runs as short as possible and as close to the PT for the power section, and near input for preamp section. Amp is quiet even without using sheilded input cable, which I will install before mounting to cabinet.
My biggest concern is the preamp wiring, trying to avoid lead dress issues. Plate resistors are the bare ones in parallel 50K (which I'll rewire tomorrow to 100K and 1.5K cathode) wired directly to pins 1 and 6.
Also, should I run the input wire from vol pot to pin 7 against the chassis instead of the height it is at now ?
This is my second build, first was a single 3 watt 6bm8 that came out well. But short on gain.Any comments, criticisms, suggestions welcomed.Thanks for the replies so far.
As I mentioned before I am using split grounds, and tried to keep the runs as short as possible and as close to the PT for the power section, and near input for preamp section. Amp is quiet even without using sheilded input cable, which I will install before mounting to cabinet.
My biggest concern is the preamp wiring, trying to avoid lead dress issues. Plate resistors are the bare ones in parallel 50K (which I'll rewire tomorrow to 100K and 1.5K cathode) wired directly to pins 1 and 6.
Also, should I run the input wire from vol pot to pin 7 against the chassis instead of the height it is at now ?
This is my second build, first was a single 3 watt 6bm8 that came out well. But short on gain.Any comments, criticisms, suggestions welcomed.Thanks for the replies so far.
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Re: 59' Supro Clone 6V6, 250 Plate voltage
Well, looking over pics of other builds, mainly 5e3's with the same vertical mounted style chassis as I am using. I need to take the plate resitors off the tube and onto terminals. Then run wires against the chassis, and if crossed run 90 degrees angles to avoid lead dress issue. Also may run heater wires to top of chassis as shown in this link http://www.celticamps.com/Aislinn.html
Will use sheilded wires for grids to pots and run them close to chassis as well. Hoping it is a lead dress issue, as that can be fairly easy to sort out.
May end up buying a turret board to help clean things up.
I still believe the PT just doesn't cut it low volts/low amps for a 6V6. I can always install a EL84 or SV83 and get a decent P1. But not giving up just yet. I definitely prefer the 6V6 sound over an EL84.
Will use sheilded wires for grids to pots and run them close to chassis as well. Hoping it is a lead dress issue, as that can be fairly easy to sort out.
May end up buying a turret board to help clean things up.
I still believe the PT just doesn't cut it low volts/low amps for a 6V6. I can always install a EL84 or SV83 and get a decent P1. But not giving up just yet. I definitely prefer the 6V6 sound over an EL84.