Pedal Steel Amp - Troubleshooting

[DWindover]

Hello all,

I'm in the process of building a pedal steel amp for a friend. It's based on a blackface Twin Reverb, but instead of four 6l6's, it has two 6550's. It also has a four knob reverb (Dwell, Mix, Treble, Bass). I designed the amp to have some of the common mods done to Twin Reverbs for Pedal Steel. When it's working, it sounds perfect. Currently, it is not.

Here are the issues:

1. The plate voltage is high in V1 (310v on pin 1, 416v on pin 6). I did not include a cathode bypass cap on the first triode, which might account for some of the issue. This is a common mod for these amps. Thoughts?

2. I'm currently getting no signal through the amp, and after about five minutes or so, it starts to feedback. Any idea of what that might be?

3. Radio signals. I'm getting some serious radio signals in this amp - picking up a few different stations.

4. Grounding - I have most of my grounds running off the buss, which is connected to the chassis at the preamp side of the board. Only the ground from the AC wire and the reverb input jack are grounded to the chassis otherwise. All other grounds hit the buss.

Your help would be very much appreciated.

Thanks,
Darcy

[xtian]

A missing cathode bypass cap is not the issue. But something is not conducting, not loading the B+ string, thus you have high voltages. Start at V1--what DC voltage do you read across the cathode resistors on pins 3 and 8?

[Stevem]

It sounds like the amp due to your added reverb stuff wiring it is oscillating due to poor wire layout ,oscillating up at a frequency you can't hear!

If you hook up a meter set for AC volts across the speaker how much voltage do you read with no signal input?
If it's more then .150 vac, or mainly above a frequency above 120 HZ you have a issue!

Part of the issue along with the too high voltage you are seing is that if you only have the regular amount of Fender twin reverb bias voltage avalable then that will not be enough to bias the 6550's right.
I am taking a educated guess that you have about 460 to 480 volts on the outout tube plates and with that much voltage 6500's will need about -62 volts of bias voltage on pin 5 of each tube to get in the ball park for idle current.

Also you should change out the Fender 2 watt 470 ohm screen resistors to 1K ohm 5 watt resistors.

You may need to convert the bias circuit into a voltage boubling type to get the needed negative voltage level and range.

Preamp cathode bypass caps will not change the idling point of the tubes and in turn how much they pull down the B plus voltage.
Preamp tube bias/ idling is set up by the relationship of the valve of the plate load resistor, cathode resistor and how strong / good the tube is itself.

What are your output tubes idling at now?

[Stevem]

It sounds like the amp due to your added reverb stuff wiring it is oscillating due to poor wire layout ,oscillating up at a frequency you can't hear!

If you hook up a meter set for AC volts across the speaker how much voltage do you read with no signal input?
If it's more then .150 vac, or mainly above a frequency above 120 HZ you have a issue!

Part of the issue along with the too high voltage you are seing is that if you only have the regular amount of Fender twin reverb bias voltage avalable then that will not be enough to bias the 6550's right.
I am taking a educated guess that you have about 460 to 480 volts on the outout tube plates and with that much voltage 6500's will need about -62 volts of bias voltage on pin 5 of each tube to get in the ball park for idle current.

Also you should change out the Fender 2 watt 470 ohm screen resistors to 1K ohm 5 watt resistors.

You may need to convert the bias circuit into a voltage boubling type to get the needed negative voltage level and range.

Preamp cathode bypass caps will not change the idling point of the tubes and in turn how much they pull down the B plus voltage.
Preamp tube bias/ idling is set up by the relationship of the valve of the plate load resistor, cathode resistor and how strong / good the tube is itself.

What are your output tubes idling at now?

When you get things working if the amp is not loud enough then you may need to tweak the PI section for more drive signal output as the 6550's do want a bit more push coming in on the grid then a 6L6.

[DWindover]

Thanks so much for your response - and sorry for my late reply. I hope to get to those tests on Monday, but it all makes sense.

What voltage divider resistors would you suggest in order to get the voltage bias to the right bias point? Currently, I've got a 470 ohm 1watt resistor (typical in twin reverbs) taking the bias voltage from the PT.

[DWindover]

Thanks for your advice! I managed to fix the amp. It was almost completely a bias issue. I looked up some of the schematics for Sunn amps and managed to play with the values of the voltage divider circuit enough to get all of the voltages on all tubes in the right place. The bias voltage is now sitting at around -66v and the B+ on the power tubes is around 450v. It's clean all the way up (which is ideal for Pedal Steel) and sounded great at our gig last night.

It's super quiet all-round, though, at the highest volume, I have a low-end hum (60 hz). Any thoughts on that one?

Also, I'm finding that the reverb section is lacking some intensity. Any ideas on how to open the flood-gates to get more verb?

[martin manning]

It's very difficult to understand what you are talking about or how the reverb might be tweaked without a schematic.

[DWindover]

Here's the schematic.

[DWindover]

The main issues now are:

1. Hum in high volumes. It's a low-end hum that comes in at around 8 on the volume. There's also a bit of high end hum in the reverb section. I have all grounds except for the mains ground attached to the ground buss, as per the Valve Wizard's site.

2. Weak reverb. The reverb should be more intense than it is.


Notes:
B+ is sitting around 450v on the OT primaries on the Power Tubes.
Bias voltage is at -66v or so.
All of the other b+ voltages are in the right range, with the first preamp around 300v (high due to lack of a cathode cap and higher cathode resister - 2.7k).

Cheers,
Darcy

[tictac]

your wet/dry mix stage is wired wrong.... look at a Twin Reverb schematic for the correct wiring

TT

[didit]

Might try this ..

pedalsteel mod.png

Reverb signal off the plate of "tonestack recovery" will be much stronger.

Best .. Ian