Twin reverb added bias pot to matching balance pot low watts per tube

[Bassden]

Hello, This Twin reverb had problems with everything so I replaced nearly everything except the chassis,transformers, eyelet boards,tube sockets,reverb tank, reverb 2.2meg resistor and speakers.Am now able to get to where the both sides of the OT are very close.
I'm using Uncle Doug's video to calculate watts out and have a very low output in watts. Signal sounds good
in both channels. Also have the one ohm resistor on all cathodes and measuring. Would like to get it up to 80 watts total.

Blu side     36 ohms    .850     377vdc      8.6 watts
Brown side 34 ohms    .820     376vdc      9 watts

Do I need to change one of the components to get a wider range of volts or current on the bias?
I do not have the exact same schematic as the twin reverb I am working on. This one I copied
is very close. What can I change to up the wattage without red plating?

[tdvt]

You need to dial in the bias range resistor (your 7.5K) to change the overall bias voltage. That is a pretty low value.

I did a similar mod to a UL Pro Reverb recently with the same bias circuit & values, thinking my range resistor was around 18K-20K

Check out this information:    https://el34world.com/charts/Biascircuits.htm

[Bassden]

tdvt thank you I will try that.

[Bassden]

tdvt, I had x2 15k resistors in parallel to make the 7.5k so, I cut one of the 15K's out, which raised up that resistance then to 15k. After that I turned on the amp and the plate current jumped up to 5 'ma instead of .023 ma. I shut off the amp right away in fear of too much current.
Do I need to reduce the 15k to maybe something around 8,9, or even 10K? I'm thinking add just 1k to 8.5k.
These are new tubes and I do not want to ruin them. Thanks for anything you can tell me.

[sluckey]

You should pull all 4 output tubes, lay them aside, and don't touch them again until you have a proper range of -45VDC to -60VDC on pin 5 of every output tube socket. The proper negative bias voltage for pin 5 will lie somewhere in this range. With the output tubes removed you can safely adjust your bias pot and balance pot without fear of damaging anything. Once you have the proper range set the bias and balance pots to put the maximum negative voltage on pin 5 of all output tubes. This voltage should be in the neighborhood of -55V.

Now it is safe to put the power tubes back in the amp and proceed to set the desired bias point, ie, mA of tube current. I highly recommend using the 1Ω resistors rather than the OT method. to measure the mA.

[PRR]

> calculate watts out
> 36 ohms    .850     377vdc      8.6 watts

You are calculating watts dissipation at idle. NOT watts audio output at full roar.

Do you know how to measure Output?

Yes, 9W diss is low, and could make the amp "hoarse" as a note fades out, but you say "sounds good".

[Bassden]

sluckey, I did pull the tubes and change the resistor to a 10k. Then I was able to get the 55vdc.I already had the 1 ohm resistors on the cathodes of each tube pin 8. Now I am measuring 10mv on each tubes pin 8. That seems low however, This is my first twin reverb so I am thinking it should be half of a 50 watt bassman for each tube so 16 ma for each tube getting 32 ma for each 2 sets? I'm not sure what to raise it to.

[Bassden]

PRR, Thank you for that info. I am saying it sounds good but that is from me who built a 100 fender amp and still need to bias it. I have built several Bassman 50 watt amps and serviced several as well replacing some old parts on the main boards and biasing with excellent results. I am getting some noise when I crank the volume controls up to 10 on both channels but this bias is not perfect yet, I cannot judge the noise on anything else yet.

[tdvt]

Looks like you are making headway.
You are in good hands, these are the guys to really help sort things out.
What year is the amp you are rebuilding?

[Bassden]

tdvt   Stamped in back chassis is A 58868 MARIE
Power Trans side says in print 23001 Dave Curtner

1968?

[tdvt]

That is earlier than I was thinking.


Maybe this is the correct schematic below? Never really saw this version, with the rather odd bias circuit.


https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab568.pdf
 

[Bassden]

I see the voltage drop resistor off the PT tap is 470 ohm in my original chassis 1k on this schematic and also the cathodes of the OT tubes have no caps on them. This is close like the one I pasted but I could not find on exact either. It all looked original to me but I do not know when the white caps on the preamp cathodes were installed. Could they be original? Again when I pull up volumes on either channel or both I get some noise, fairly loud. I do not know if that will be corrected with I get this bias right.Wish I had a scope to trace the noise.Picture is all original. I replaced everything on the main board except the 2.2 meg resistor

[tdvt]

Schematic I linked isn't correct as you have a MV, but it doesn't look like it has the push-pull boost switch on the back of the MV pot (which was mid-late 70s).

Early 70s maybe? That must have been quite a project swapping out the board, I am about to undertake that myself on a hopelessly waxy Pro Reverb board.


Try this schematic, not labelled specifically as a "Twin" but by output. Has the Twin speaker wiring included as an option.

  https://el34world.com/charts/Schematics/files/Fender/Fender_quadreverb_100w_nomstrvol.pdf

[Bassden]

I believe that is it tdvt. Only difference I can see is the 6L6GC cathodes on my chassis were individually grounded to chassis, this shows them tied together and grounded I don't believe that is any big deal.
Thank you for that.
I have a new Bassman 100 built, pic below and plan to use what I learn here to bias it properly. Cabinet was from another amp so it's not a perfect copy although I made it taller and able to get the output tubes out easy. I also made it with a screw top removal for easy service.
I also made 2 other Bassman amps with the AA864 schematic, one with an addition of a Marshall channel change using the not used second 12AX7 B portion and one without. Just fun building. I've been working with Kyle Mathis of Musician Madness out of Crowley, Texas. He's going to try to sell them when I'm done.This is his site. https://www.musicianmadness.com/

[Bassden]

As I lower the resistance of the 7.5K ohm voltage divider on the bias circuit to 6.7K I then get a higher mv to 26mv with the new bias pot maxed out. At 7.5K ohm I was getting about 10mv maxed out on each cathode across a 1 ohm resistor.
Do I need to continue lowering the bias voltage divider to get to some higher mv reading?
I am somewhat confused as in reading all the responses I thought I would be upping the resistance to achieve a higher mv reading. I know that the best reading on a Bassman 50 watt is 32 mv across a 1 ohm resistor. Again, I have no clue as to what this reading should be on this twin reverb.
No red plating as I go through all of this.
On this video I notice she is getting a 32 ma reading balancing without a bias pot.

[Bassden]

To all or anyone,
I've got finally a 3.3K ohm resistor in the Bias of the 10K pot and it looks like I'm in range.
My plate voltage is 275vdc and I have .050mvdc on the cathode. That gives me 13.5 watts.
At this point I have some noise as it got louder as I decreased the ohms on the bias resistor,
when I crank any of the volumes up with the master up.
After watching the video above, I'm somewhat reluctant to increase the bias to get to 20 watts each.
Not sure where to go from here. It is like something else is not right.

[sluckey]

My plate voltage is 275vdc and I have .050mvdc on the cathode.

Something is sick if you are correctly measuring the plate voltage with a good meter. You should have 450VDC, maybe even more. You should start looking at the PT and power supply.

[Bassden]

I removed the output tubes and I get 460vdc on the plates. Everything is new except the PT, choke, OPtube sockets, boards,
Something is pulling down the 460vdc Meter has always worked good. On it's second battery,..About 2 years old. I will try some resistance measurements. Picture is what I have

[Bassden]

Is it possible to install one 6L6 at a time or one side with 2 6L6'a to maybe see which side is pulling down the HV? Even one at a time until all are installed?

[Bassden]

Wow, Swapped the PT from a Bassman 100 new PT, I have 330vdc now on the plates. Something still pulling this voltage down. Even tried the tubes from the Bassman 100, same thing. Sure a new part can be bad. Only happened twice in my life time. Is it possible the Choke or OT could be bad only when load applied? Both measure ok with meter. Traced out the power supply caps and resistors several times. All connected good there,. Any other suggestions? I'm still thinking.

[Bassden]

I began rethinking this 2 bias pot thing. I eliminated the extra pot for bias and changed the balance pot to a bias pot via these instructions https://www.premierguitar.com/gear/ask-amp-man-better-biasing-for-silverface-twin-reverbs
When I did I got my plate volts back to 460 vdc. I'm only able to dial the pot in for 39 mv on the cathodes.
Should I change the 15k resistor to get it in range? Thank you anyone.

[shooter]

I removed the output tubes and I get 460vdc on the plates

plate PINS?


try this;
NO PA tubes, standby switch closed, meter Vdc PS Tap A to chassis, power on, wait 10 seconds
power OFF and measure time for meter to discharge to <20 vdc
Now;
NO PA tubes, standby switch OPEN, meter Vdc PS LEFT side standby switch to chassis, power on, wait 10 seconds
power OFF and measure time for meter to discharge to <20 vdc


discharge time close to each other?

[Bassden]

Shooter, I changed the biasing from using a balance pot and an extra pot for bias to changing the balance pot into a bias pot and that solved the not getting 460vdc on the plates.. Im using this bias now https://www.premierguitar.com/gear/ask-amp-man-better-biasing-for-silverface-twin-reverbs
What I need to know is can I change the 15k now to bring the biasing range in? I' never done this on a twin reverb. I've built several Bassman 50 watt AA864 amps with no biasing problems. This balance pot is not a good thing. CBS was really screwing things up putting those on amps.

[tdvt]

I didn't think to mention it earlier, but the stock balance pot in that amp is an odd tapped pot (4 terminals) & the adaptation might have had unexpected consequences as a result.

I did not use that pot & rebuilt the bias/balance circuit as on the right-hand side, below (not my drawing) using 2 regular trim pots. Thinking I started with the values shown on the drawing & adjusted the value of the 22k off the wiper to achieve a usable range.
 

[Bassden]

tdvt Thank you. Seems like the Dual Bias would be really good for mismatched tubes.
It's looking like everything I get to work on here has mostly all original old tubes that are
not all that good so I usually replace them with the new JJ's and have had good luck.

I changed the bias to what I described turning the balance pot into a bias pot then checked
my plate volts and it is back to 290 vdc. The only thing I have yet to change out is the OT.
Going to try that and come back. I have not found yet what would cut the PS volts nearly in half
under load.

[Bassden]

Plate voltage up to 300vdc with a new OT. I got something pulling it all down. New items are bringing up the voltage. Dog house parts are being looked at. I keep saying, "Which one of you is pulling the voltage down"?
Possibly under the main board too much solder. Looking.

[tdvt]

I changed the bias to what I described turning the balance pot into a bias pot

Have you tried a new 3-terminal bias pot?

[Bassden]

TDVT, It measures 10k but no, have not replaced it yet.I shall try.

[Bassden]

TDVT, You are TOP SHELF TOP TECH !!!!  Thank you, 460vdc on the plates and 53mvdc across the 1 ohm resistor fully adjustable by the new 10K pot. Hot Diggity, Deputy Dawgs. We are in business. Now I gotta go back and remove the PT, choke and OT and install the old ones LOL Thank you TDVT!!!!

[tdvt]

Glad you got it!

[Bassden]

Well I thought I it. LOL.
Since the replacement of the bias pot everything was working besides a dead spot in the master vol pot.
I replaced that pot and that was fixed.
Then I went ahead and returned all the transformers back to the original.
Low plate volts again on the output tubes at about 240vdc
So I replaced the original back to the new transformers and now plate voltages are going below 200 vdc
with my over current bulb very bright. Even replaced that master vol pot again. 2 Speakers in parallel are 4 ohm.
The only thing i have not replaced are all the other pots and the boards. I've searched for a possible short underneath the boards and lifted them a bit just in case I don't see anything shorting to the chassis.There's a ghost load in this chassis somewhere. About to call Ghostbusters.Anyone have any ideas, I will try anything at this point.

[tdvt]

Sorry to hear that wasn't the fix. If it was my project, I think I would start to divide & conquer.

Not sure how waxy your particular board(s) was/is, but bottom line is the boards can become contaminated enough that they are unusable. Either extreme cleaning or replacement are your two options, & rebuilding in any event. After screwing around with a few late 70s boards I am inclined to replace them.

Seems like if you start with rebuilding the power supply (r&r the cap board & bias board & power rail), testing as you go, then you will KNOW that you are not leaking voltage from those sections & you can move onto the main board to find the stray voltage.

[shooter]

reply 21 was to 1/2 split the PS

[Bassden]

tdvt, that is my thoughts, trying to avoid all the work but I don't think I have a choice.

shooter   I did that when you said and saw no difference in timing. Since then I had removed all the PS filtering and metered all good and the put it back. Sorry I did not respond earlier. Got busy with dr, appointment and pay day grocery shopping for the month.

[shooter]

try this;NO PA tubes, NO power, standby switch closed, meter OHMS PS Tap A to chassis

same setup OHM PA grid pins at tube side of sockets to chassis.
post values
Tap A
Socket 1
Socket 2


was the discharge quick seconds, or lots of seconds?

[Bassden]

shooter
Tap A of the power supply, top of cap? both before and after choke is infinite.
Socket 1 pin2 1484 ohms
Socket 2 pin2 1486 ohms

This bottom statement i do not understand with no power on
was the discharge quick seconds, or lots of seconds?

[Esquirefreak]

Is only the HT winding being brought down or are the other (like filament) voltages low too? I was thinking if that mains voltage selector switch could have an intermittent short.

EDIT: Scratch that part about the switch, I was thinking about the export models. Sorry.

/Max

[shooter]

I did that when you said and saw no difference in timing.

the caps got charged ^^^here, or should have.
once power is off, they bleed down, typically many seconds, an open to ground they bleed slower, a near short to ground the bleed real fast.


EDIT:
i'm using the schematic TDVT posted;
G1 is on pin 5
there are 4 tubes, 4 readings
adding up the G1 to ground R's gets ~~~ 73K, might be parallel paths I missed, it's early

[Bassden]

Esquirefreak Yes the filament voltage also goes down as well. This amp though is a US only.

Shooter It is bleeding faster then say a Bassman AA864 twice or more faster.

While those were noticed, this morning I replaced the diode board using new diodes with no change. Next I will change the power supply filter dog house board if that does not work then the main board, I have to order those so I shall return at some point.

If I go as far as replacing the main board, What schematic with a wiring diagram would be best? or what is popular should do this rebuild to? AA763? Any suggestions on the best for a Silverface twin reverb outside case? Blackface it?

[Esquirefreak]

Carbon traces on (or dirty) tube sockets?

/Max

[Bassden]

Esquirefreak All cleaned initially. I do a bunch initially cleaning mostly but who knows as old as this is looks like 1968. Fired in the contact cleaner to the sockets and pots. Also, I cleaned the chassis as best as I could like this amp was being used next to a mop solution of something liquid, as the chassis is foggy looking over all.

[Bassden]

I am going to rebuild this twin reverb to Rob Robinette's Blackface version here:
https://robrobinette.com/AB763_Modifications.htm#google_vignette

The twin reverb is close to the bottom of this page.

[tdvt]

If I go as far as replacing the main board, What schematic with a wiring diagram would be best? or what is popular should do this rebuild to? AA763? Any suggestions on the best for a Silverface twin reverb outside case? Blackface it?

You have a few options if you decide to replace the board;

Use Doug's AB763 turret board, incorporating the Twin Reverb options (mid control, 4 power tubes, SS rec, etc.) The original caps won't readily fit without some modifications, if you are planning to reuse them.

https://el34world.com/schematics.htm#Hoffman_AB763_Two_Channel_


Hoffman, Mojotone, Ebay, etc have fiberboard replica eyelet boards available

https://hoffmanamps.com/MyStore/catalog/EyeletBoard.htm


Make you own from fiberboard or G10 or have Doug make a custom board.

[Bassden]

tdvt Thank you

[sluckey]

I am going to rebuild this twin reverb to Rob Robinette's Blackface version here:

I strongly suggest resolving the issues with this amp before rebuilding. Or at least determine for sure what the issue is.

[tdvt]

I strongly suggest resolving the issues with this amp before rebuilding. Or at least determine for sure what the issue is.

I think this is very good advice.

[Bassden]

Sluckey and tdvt
I agree as well. I want to pull the main board and the dog house board out to see them. I have already replaced the diode board which had a pin hole in it but nothing good as a result with new. If I desolder everything to do it, I will certainly rebuild since that part is done. I do not have a problem with all the extra work. I have ordered the tube sockets, the boards and pots. Somewhere in all of it is a short to ground I cannot see or measure yet. Digging into it this deep should put me closer to the problem anyway. I'm retired and have the time and do this repair now and then to help a friend who has a store.This was a trade in on new. I have built several AA864 Bassman's so I'm familiar with it. It's a learning journey. LOL.

[sluckey]

Somewhere in all of it is a short to ground I cannot see or measure yet.

A short to ground is very easy to measure and should not be too difficult to physically locate in a simple TR circuit. A partial short can be more difficult to find but if you have a good understanding of circuits you would have an idea of what resistance to expect and go from there.

[Bassden]

As soon as I find the problem, I will let y'all know.

[tdvt]

When you de-populate the cap board, scrape any bulky wax off, then heat-gun/paper towel off what you can liquefy on both sides, then thoroughly soak & scrub with isopropyl alcohol a few times to get it really clean.

Same goes for the insulation board underneath which should be a good bit less funky.

Keep an eye out for any solder blobs & wire scraps that might be hiding underneath & causing your issues.