6G6a PI voltages

[pontiacpete]

Still fooling with this amp;

put in a PT from a twin and changed the OT to a new sensor by EH with a multi tap and using the 4 ohm connection w/ a 4 ohm spkr.
This is what is happening

440vdc on the plates
-51 on grids
45mA
25W (used the method of measuring ac at speaker terminals)
seems low for this amp. No?

How can I get the B+ up to the 500v range?

Another question: what is it with the tone controls on this amp, they seems to sound good with them all rolled back to practically 2 on the both pots, bass and treble?

[MWJB]

"How can I get the B+ up to the 500v range?" Put the old PT back in.

"Another question: what is it with the tone controls on this amp, they seems to sound good with them all rolled back to practically 2 on the both pots, bass and treble?" This doesn't sound right, post some pics of the pot wiring. Same on both channels (Normal is just a regular tolex style channel)?

"440vdc on the plates
-51 on grids
45mA
25W (used the method of measuring ac at speaker terminals)
seems low for this amp. No?" - Did you use a signal generator, or play through the amp? List dc voltages for all tubes at idle.

[pontiacpete]

I played through the amp, but I must correct myself, meant to when the internet went down---- I played and got about 15vac across the speaker, so that's about 56w. I guess that's ok.
I was wondering about the B+.
I have another thread started about tube/speaker proximity. Also wondering if the ratty sound I'm getting is due to the 6550s too close to the speaker.

I'll check voltages on all tubes later when I get home.
thanks

[MWJB]

"I played and got about 15vac across the speaker, so that's about 56w. I guess that's ok." At that B+, I'd expect around 60W-ish, so it seems ball park.

"Also wondering if the ratty sound I'm getting is due to the 6550s too close to the speaker." How close are they? Describe the ratty sound.

[pontiacpete]

oh,, I'll also post some pix.

[pontiacpete]

the tubes are about an 1 1/2" away for the magnet.

THe sound is a buzzy trailing off sound on the low notes.
THe wave has a hump on one side of the downward slopes.

[MWJB]

"the tubes are about an 1 1/2" away for the magnet." Doesn't seem too bad.

See if you are getting unbalanced AC signal voltages at the 6550 grids.

[pontiacpete]

here are some pictures, from left to right goes as follow: bass, treble, volume, input jack and then presence(this could be one problem, no?)
I put a 250pf across the volume pot in an attempt to quiet down the amp, also did that across pin 3 and 1 of v2. There's 100pf silver mica that is snipped at one end, had tried that, now removed.
I change the NFB resistor from 56K to 22K. Helps a bit.
Also , change the 56K in the bias supply to 22K , which brought the grid more positive from -67v to the 51v.

The voltages are as follows:
v1
Pin 1) 127v
3) 1.2v
6) 227v
7) 127v
8) 127v,,, is that right?
v2
Pin 1) 139v
3) 1.2v
6) 144v
8) 1.2v
v3
pin 1) 248v
3&8)42v
2) 30v
6)243v
7) 32v
v4
pin 3) 445v
4) 433v
5) -50v
6) 443v
v5
pin 3) 443v
4) 436v
5) -51v
6) 443v

thanks for looking.
pete

[MWJB]

"v1 pin 8) 127v,,, is that right?" Yes, it's a cathode folllower.

"v3 pin 1) 248v" not a 12AX7 in V3 then?

Boy, you really have crammed things in there tight, it's not really a good idea to squeeze everything close together like that. Those OT secondaries could be re-routed to enter the chassis nearer to the power tubes, but I don't know if it will ever work perfectly with that layout?

[pontiacpete]

"v3 pin 1) 248v" not a 12AX7 in V3 then?
that's the 12at7

Boy, you really have crammed things in there tight, it's not really a good idea to squeeze everything close together like that. Those OT secondaries could be re-routed to enter the chassis nearer to the power tubes,

I'll give this a try, thanks

[pontiacpete]

running the OT wires under the board helped,,,thanks for pointing that out to me. Should have thought of that myself.
This amp is an attempt at making a compact tube bass amp, removing one channel and cramming it into a small chassis was a risk I took. There may still be some vibration issues present.
I was hoping to give this to a friend whose amp I broke trying to fix, a polytone 1 X 15" combo.
Thanks again, greatly, for all the help. If is doesn't work as a bass amp it will as a guitar amp.

pete