50W Plexi, Dual Rectifier, PPIMV Budget Build

[Guitarnut]

I played the amp a bit more last night and I noticed that with the volume turned completely down on the amp, I still get just a bit of sound thru the amp. It happens on both channels. What might be causing this?

Peace,
Mark

[martin manning]

Conductive fiberboard? Seems to be getting more common... Where did your board material come from?

[Reeltarded]

Oh no that would suck. Check grounds on volume pots and gain stages too though.

[Guitarnut]

Conductive fiberboard? Seems to be getting more common... Where did your board material come from?

The board is the original from my Bassman build...it came from Mojotone.

[Guitarnut]

Oh no that would suck. Check grounds on volume pots and gain stages too though.

I reflowed all the soldered grounds and it didn't make any difference.

[Guitarnut]

I hooked up the amp to my 8Ω Line 6 cab last night and played a bit. My lord, that thing is ear splitting loud! I have to get the MV back in the mix.

I have a heavily modded Fender Lead II with a very high output HB pickup...Carvin M22SD...and this amp really screams. I had to repair a 20' cable so I could far enough away to control the feedback. Not off-the-wall screeching feedback but the smooth, silky kind after sustaining chords or notes. The low end of the amp is really nice and the tone stack seems fairly flexible.

I have to say, with this circuit, I love the SS rectifier. It's responsive, tighter and at full volume the amp is "creamier"...if that makes any sense.

I'm gonna work on the cab today and maybe record a bit with the Line 6 cab. I'll also get a set of voltages for each rectifier.

Peace,
Mark

[Guitarnut]

I forgot to mention, I swapped the PI leads and restored the NFB and it's all good. Over all, the amp is very quiet until I get to about 9 on vol of either channel. Then I start getting what sounds like thermal noise...really just sounds like a whole lot of guitar about to happen.

Peace,
Mark

[martin manning]

Conductive fiberboard? Seems to be getting more common... Where did your board material come from?

The board is the original from my Bassman build...it came from Mojotone.

See here: https://tubeamparchive.com/viewtopic.ph ... 239#258239 I'm not sure this is the cause, but it's a possibility.

[Guitarnut]

I reinstalled the MV and the amp is much more playable in a bedroom setting. It still has some nice grit and harmonics at about 25% volume. It seems to have solved the vol pot issues as well. When the MV is all the down, I can no longer hear the guitar thru the amp as before. Not sure that's the right fix but it's better than replacing the board.

The Line 6 speaker is rated at 50W but if I push it, it starts farting pretty good. The 70W Eminence should handle the power better.

I also added the tube retainers and did a last check on the solder joints and double checked for any stray wire strands and solder blobs. I did a much nicer job with my soldering this time. The lack of splatter and blobs bears that out.

My next post will have voltages and bias settings.

Peace,
Mark

[Guitarnut]

See here: https://tubeamparchive.com/viewtopic.ph ... 239#258239 I'm not sure this is the cause, but it's a possibility.

Thanks Martin. Before I seal it up, I'm going to check for current between eyelets and see what's there. I'll also check for any DC leakage at the pots.

Mark

[Guitarnut]

I just ran the numbers and I see now why I prefer the SS to the tube. My power tubes aren't biasing properly on the tube rectifier.

The 6L6s are drawing 39mA on the SS and only 17.5mA on the tube side. They're barely on at all. When I use the bias adjust, it only gets worse.

Not making sense to me. Do I need to increase the 43K R I upped from 15K earlier?

Peace,
Mark

[Guitarnut]

Solid State Rectifier - Master Volume Full

F1=445v
F2=445v
F3=315v

V1
1 151v
3 .853v
6 196v
8 1.63v

V2
1 154v
3 .936v
6 272v
7 154v
8 155v

V3
1 212v
3 35.5v
6 209v
8 23.5v

V4
3 443v
4 441v
5 -42v
6 443

V5
3 441v
4 440v
5 -42v
6 441v

[Guitarnut]

GZ34 Tube Rectifier - Master Volume Full

F1=397v
F2=397v
F3=279v

V1
1 135v
3 .853v
6 174v
8 1.63v

V2
1 137v
3 .836v
6 242v
7 136v
8 138v

V3
1 191v
3 31.5v
6 187v
8 31.5v

V4
3 397v
4 396v
5 -43v
6 396v

V5
3 398v
4 398v
5 -43v
6 399v

[CHIP]

The GZ34 is dropping your B+ @50 volts, which is normal. So maybe you'll have to put a swtichable bias supply for each type of rectiifer. I'm not certain how to do that off hand , but I'm sure with a little research you could find out, or help from Martin or one of the other very knowledgeable guys around here.

[Guitarnut]

Here's my thinking on this.

On Martin's advice, I raised the 15K R in the bias circuit by 28K to a 43K R. This gave me a 10V hotter bias voltage.

Current bias range.
SS -57.5v to -41.5v
GZ34 -55.3v to -43.5v

If I go up another 25K to 63K, would that give me another 10V bump?

If so, this would be the new bias range.
SS -47.5v to -31.5v
GZ34 -45.3v to -33.5v

This would get me the -42v where the SS is happy and allow me to dial in a hotter bias for the GZ34. I'm thinking I could even leave the amp biased for the tube and then add a R at the switch to limit the bias for the SS rectifier.

But, this is all assumption at this point.

Peace,
Mark