[_Knighthawk_]

Yes. A couple. High squealing when I try to bias - even at 60% dissipation (have the bias pot maxed at the moment). Squealing stops when I remove the output preamp tube (V6 AT7). Can't even plug in the reverb at the moment or she squeals like an electrified hog. I'm also getting severe distortion in both channels when I turn the volume past 3 or 4. I've checked the circuit. Swapped out some bypass caps and verified every resistor on the bord. Voltage is the only issue I can see.

 

[Paul Running]

when I try to bias - even at 60% dissipation (have the bias pot maxed at the moment

That is understandable with 484VDC on the screen-grids of the power tubes.

 

[Paul Running]

Voltage is the only issue I can see.

You could replace the GZ34 with a 5Y3 to drop the voltage and see if it still squeals.

 

[_Knighthawk_]

I will check to see if I have one kicking around.

 

[Alan Small]

Yes. A couple. High squealing when I try to bias - even at 60% dissipation (have the bias pot maxed at the moment). Squealing stops when I remove the output preamp tube (V6 AT7). Can't even plug in the reverb at the moment or she squeals like an electrified hog. I'm also getting severe distortion in both channels when I turn the volume past 3 or 4. I've checked the circuit. Swapped out some bypass caps and verified every resistor on the bord. Voltage is the only issue I can see.

nb. still possible that reversed feedback signal would cause the distortion and squeal if all else is good...i have experienced similar

 

[_Knighthawk_]

nb. still possible that reversed feedback signal would cause the distortion and squeal if all else is good...i have experienced similar

It's worth investigating. Where would I start?

 

[Alan Small]

It's worth investigating. Where would I start?

we discussed this two pages back I think...
disconnect the yellow wire

if better results then switch green and black wires and reattach yellow same spot

if no change put it back and look elsewhere

 

[Paul Running]

As a side note when, I experimented with 60s, 6V6GTAs in push-pull with near 500VDC plates, screen-voltages over 250VDC, produced instability; I believe that it may be worse with modern 6V6's...they do have a design maximum rating of 315VDC for the screen.

 

[_Knighthawk_]

You could replace the GZ34 with a 5Y3 to drop the voltage and see if it still squeals.

Don't have a 5Y3. I have a 5U4GB but it has a different amp draw.

EDIT: Found a 5Y3 (had one in my 5E3).

Is this rectifier tube okay to use with this PT? I note that the current draw for the 5U4 is 1.9 amps while the 5Y3 is 2.

 

[Paul Running]

At 100mA load, the GZ34 drops 8VDC per plate, 5U4 drops 33VDC per plate and the 5Y3 drops 50VDC per plate so, the 5Y3 will provide the greatest voltage drop.

 

[_Knighthawk_]

At 100mA load, the GZ34 drops 8VDC per plate, 5U4 drops 33VDC per plate and the 5Y3 drops 50VDC per plate so, the 5Y3 will provide the greatest voltage drop.

I was using this chart:

Tube Type	DC mA	PIV	FIL AMPS	Max ACV	DCV	DCV DROP
Diode Rectifier					425
5AR4 / GZ-34	250	1500	1.9	450	415	10
5V4-GA	175	1400	2	375	400	25
GZ37	350	1000	2.8	450	388	37
5U4-GA	250	1550	3	450	381	44
5U4-G	225	1550	3	450	381	44
5U4-GB	275	1550	3	450	375	50
5Y3-G/GA	125	1400	2	350	365	60
5R4GYB	250	3100	2	900	362	63
5R4G/GY/GYA	250	3100	2	750	358	67

 

[Paul Running]

I have a 5U4GB but it has a different amp draw.

According to the specs on the 291BX, it will handle 3A for the 5VAC winding; you could try the 5U4.

 

[Paul Running]

 

[Paul Running]

Do you know somebody with a scope? A scope will help locate the origin of the parasitic oscillation.

 

[_Knighthawk_]

5Y3 is in and the voltages have come down to reasonable levels. That said, if I tweak the bias up a bit warmer, the squeal is still there so I clearly have something else going on. I note also that when I plug in the guitar and strum, the sound goes staticy/scratchy past 3 on the volume dial on both channels.

 

[_Knighthawk_]

Do you know somebody with a scope? A scope will help locate the origin of the parasitic oscillation.

HA! You mentioned a scope before.

I have a scope! AND I bought the probes for it. Unfortunately, I have only gotten as far in my lesson as learning how to calibrate it. I also don't yet have an isolation transformer (nor do I know what I need one for).

 

[Paul Running]

I

sound goes staticy/scratchy past 3 on the volume dial

Does the scratchy static occur right away or is it a gradual increase as the level passes 3?

 

[Paul Running]

I also don't yet have an isolation transformer (nor do I know what I need one for).

You will not require the isolation transformer, if the scope has hydro ground (3-prong plug).

 

[_Knighthawk_]

It does not occur right away that I can detect with my ears). It occurs only as one turns up volume.

 

[_Knighthawk_]

You will not require the isolation transformer, if the scope has hydro ground (3-prong plug).

It has a 3 prong plug (it is old - but not that old). I note also that it has a fuse holder in the back.

Is there an online resource that you could point me that might assist me in learning how to use for troubleshooting amplifiers?