AA electronics PCB #183 build, clip added

[Bob-I]

This has been a start/stop project over a year in the works. First off, kudos to AA electronics for their PCB quality. I built a Super Reverb using their board awhile ago and it’s excellent.

Today I finally got everything working although I’m not happy with the OD yet. I found one confusion on the PCB markings. There are 2 pads next to each other, one labeled “vol” and one labeled “drive”. The drawing is accurate but the labels are confusing. The “vol” actually goes to the drive and “drive” goes to OD volume, had me stumped for awhile.

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Here’s a pic, I’ll post clips later and ask for input on tweaking the OD.

[Bob-I]

Adding a quick clip.

I'm not unhappy with the clean, but the OD is not Dumble at all, very wooly and muffled.

Where to start?

FYI,
Plate 445
Bias 42ma
trimmer 23k to ground
V2 voltages seem in range, ~185/1.5

[67plexi]

1 What output transformer did you use ? should be 3K2-3K5 primary plate to plate for a 50 watt
2 you need to adjust your dropping string V1 and V2 pin 1 and 2 should be 200 volts
3 on the PI should have 5-7 volt differential
4 on the 50 watt I changed the 150K 220K resistors on V1 to 135K 135K
5 on the bias I use 470 volts 34 mA

[67plexi]

Sorry pin 1 and 6 should be 200 volts on V 1 and V2

[Bob-I]

1 What output transformer did you use ? should be 3K2-3K5 primary plate to plate for a 50 watt
2 you need to adjust your dropping string V1 and V2 pin 1 and 2 should be 200 volts
3 on the PI should have 5-7 volt differential
4 on the 50 watt I changed the 150K 220K resistors on V1 to 135K 135K
5 on the bias I use 470 volts 34 mA

OT is probably 20 years old, Classic Tone, sold as a Fender replacement for 50 watt amps with 4-8-16 ohm taps. I don’t have the specs. I used it in an AB763 type amp I build years ago and it sounded great. I think I remember it having a 4K primary. I’m running the 8 ohm tap into a 2x12 with 16ohm WGS veteran 30 and retro 30.

I’ll tweak the dropping string, check the PI. I don’t have any 135k resistors, you’re the first I’ve heard of using them.

I’ll adjust the bias, but I don’t think I’ll get 470v out of the PT, we’ll see when I lower the bias.

Thanks for the input.

[martin manning]

What values are your dropping string resistors now, and what are your V1 plate voltages?

PI plate voltage difference is meaningful if you have a balanced PI tube. Otherwise you should AC balance the PI at the power tube cathodes.

If you have the Classic Tone part number I can look up the specs for your OT.

[67plexi]

Is the output transformer a 40.18007 4.2 primary 2 ohm secondary

https://s3.amazonaws.com/tubedepot-com- ... 1487777152


i use a Dagnell 784-139 3K3 primary 4-8-16 ohm secondary for my ODS 0 EL 34 builds sounds great.

[Bob-I]

What values are your dropping string resistors now, and what are your V1 plate voltages?

PI plate voltage difference is meaningful if you have a balanced PI tube. Otherwise you should AC balance the PI at the power tube cathodes.

If you have the Classic Tone part number I can look up the specs for your OT.

The dropping string, after the choke is 3k, 22k, 2.2k

V1 plates are both around 180

The sticker on the top of the OT says 744 9960338

Thx

[Bob-I]

Is the output transformer a 40.18007 4.2 primary 2 ohm secondary

https://s3.amazonaws.com/tubedepot-com- ... 1487777152


i use a Dagnell 784-139 3K3 primary 4-8-16 ohm secondary for my ODS 0 EL 34 builds sounds great.

The OT has 3 taps. My note (from years ago) has them at 4-8-16.

Oh, and I have 6L6s in it now. I can switch to EL34s if it makes sense. I’m not sure what tap to use though.

[martin manning]

Can’t tell anything from the 744 part number on the OT It’s not the usual 40-18XXX format they used. The 9960338 is the EIA code, 38th week of 2003. If it was for a 50W AB763 odds are it has 4k or 4k2 primary.

180V on V1 plates is not a bad place to be, but you could raise it a bit by reducing the first dropping resistor to as low as 1k.

[Bob-I]

I adjusted the bias, minimum I could get is 35ma, and measured voltages

6L6 plates - 454
Screens - 452
Bias - 35ma
V1a - plate 180, cathode 1.6
V1b- plate 184, cathode - 1.62
V2a - plate 196, cathode - 1.5
V2b x plate 201, cathode 1.43
V3 plate a - 235 plate b - 232.9, cathodes 78.5

[Bob-I]

Can’t tell anything from the 744 part number on the OT It’s not the usual 40-18XXX format they used. The 9960338 is the EIA code, 38th week of 2003. If it was for a 50W AB763 odds are it has 4k or 4k2 primary.

180V on V1 plates is not a bad place to be, but you could raise it a bit by reducing the first dropping resistor to as low as 1k.

It may not be a Classic Tone, I wa just guessing.

[martin manning]

EIA 996 is Heyboer. Classic Tone (Magnetic Components) was 166. I should have checked that.

Is that 35mA idle current measured at the cathodes? That seems a little on the cold side at 454x.035=15.89W. You might try increasing it to 18W.

[Bob-I]

EIA 996 is Heyboer. Classic Tone (Magnetic Components) was 166. I should have checked that.

Is that 35mA idle current measured at the cathodes? That seems a little on the cold side at 454x.035=15.89W. You might try increasing it to 18W.

Yes, measured across a 1 ohm at the cathodes. I thought it was pretty cold, but 67plexi suggested it above.

[martin manning]

18W Pa would be 60% of 6L6 30W Pa max, but about 5% of that is screen current so really it’s more like 55%. Still on the cool side. If there is no difference in the tone then that isn’t a problem.