Output Transformer Question

[ajeffcote]

Just finished reworking a Fender DRRI,hand wired on a turrett board.Hooked it up,powered up,checked voltages.Everything was as it should be.I flipped on the standby switch to adjust the bias and further check the voltages.After a few minutes I realized I had not plugged the speaker cable in.I plugged it in and flipped the stanby on only to hear this crazy motorboat sound coming from the speaker.I turned it off and checked the B+,center to either end.One end reads 134.6 ohms and the other reads 120.4 ohms.Did I just waste my OT?

[Amp Kat]

No I don't think so. Being that you heard sound at all is a good thing. It's when you don't hear anything at all is when it's a bad thing. Check across the windings now or across both leads going to each tube not from CT but outer leg and it should read about 254 ohms which is kinda high but could be. Sounds like you have a bad connection and my guess is an open ground on the filter caps. Check all of your grounds especially on the supply and make sure there is continuity from there to other grounds or ground itself.

[trevorus]

Switch your two OT wires that go to the plate (iirc) Should be brown and blue. I have heard that that can cause this, and truthfully, I don't know why...

[Jag]

No that would cause high squealing due to positive feedback. I think Amp Kat has it right.

[ajeffcote]

I removed the OT in question and installed another(the original Fender OT)and the amp works,no motorboat.The OT In question,still shows about a 16 ohm difference between the ends and CT.End to end is about 254 ohms.
Should not the windings from CT to end be about the same?
I did have a loose(kind of loose)ground connection for the screen supply cap.
Also have other problems.The vibrato won't work.Reverb works great.Both treble pots,normal channel and vibrato channel do nothing to adjust the tone.The bass pots work and the mids sound about right(via a 6.8K resistor,no mids pot).I bought the board,a Hoffman turrett board,unpopulated and soldered everything in place myself per the Hoffman diagram and instructions.I have checked,rechecked and triple checked everything and all looks right.
Where do I start looking now?

[lowell]

no both sides of the OT primary are not going to measure equal resistance in ohms... reason being that whichever side of the center tap was wound last has more wire since the circumfrence of the core is then larger due to the initial wire that was wound before it. So both sides have the same # of turns but not the same resistance. There is a tutorial on geofex.com for testing OTs. I suggest you try those, I've learned alot from that tutorial.

Do you have a new vibrato tube in? Is the footpedal plugged in and on for the vibrato? If yes have you checked every component per the schematic around the vibrato tube? Can you do a voltage check on the vibrato tube pins? What do you find?

As for the treble pots, the only thing I can think is that something is wired wrong in the tone stack. It wouldn't seem to be a tube or other part of the circuit being that the rest of the tone stack works and that you ARE getting signal out of that stage. report back what you may find.

[ajeffcote]

New tube,and have tried different tubes.
Foot pedal plugged in and turned on.
I have checked all compnents.
I won't have time to check voltages until tonight after work.
The tone stack is wired correctly.I did find a mistake on the input jack wiring.I forgot to ground the switch on the #1 jack of both channels.But that should only disable the voltage divider there,right?Definitely getting signal,but did not notice a difference in gain between the 2 inputs.
I used shielded mini cable for the inputs from both normal and vibrato jacks,and for the grid wire from the volume pots back to pin 7 of V1 + V2.Maybe the mini cable's capacitance is enough to siphon off the top end?When I check the voltages tonight I am going to replace that mini cable on the normal channel to see if that makes a difference.
Thanks Aubrey

[lowell]

Aubrey,
So the treble doesn't work... is it permanently "down" as in no treble coming through, or permanently on "high." Maybe you checked it but make sure not only that the pot is wired right, but that the tone caps in the stack are wired right. Just wanna make sure of this.

[ajeffcote]

I think I made a mistake in the tone stack wiring.And i think I have corrected it.
The rec tube has 354V AC coming in and 492 DC going out.This is with the Merc Mag OT re-installed.I have -60V available in the bias circuit.Do I have too much plate voltage for the Fender?Groove Tubes 6V6GT output tubes?
When I switch the standby on,the bias voltage,measured across 1 ohm res hooked to pins 1+8 and ground,takes off and I immediately have to shut it down.And I get a rapidly rising pitched scream from the speaker.
I have checked the filter grounds and they all seem ok..3 -.4 ohms max.
Got to go to work.I will check back tonight.
Thanks to all of you for trying to help out a newbie.

[Gtr_tech]

Switch your two OT wires that go to the plate (iirc) Should be brown and blue. I have heard that that can cause this, and truthfully, I don't know why...

It is due to a feedback loop, if the amp has one. See, the FB loop is s'posed to supply negative feedback (out of phase with the signal at the point it is added). If the primary (or secondary) is wired reverse polarity, you get POSITIVE feedback that causes oacillation.....turning your amp into an air raid siren. Amps without the feedback loop don't care about output xfmr polatiry.

[lowell]

As far as your cathode measurement skyrocketing yeah, you might have the blue and brown wires swapped.

Yes 492vdc is too much for those tubes. That's probably why the voltage is spiking like that. I guess technically you could use that voltage but you'd need to really limit the plate current, by increasing the bias quite a bit. Double check this option as I'm not 100% positive. 492vdc is nearing the high point for even 6L6s which have a higher max plate watts dissipation. All in all you should become familiar w/ plate-current and its relation to plate-volts. Multiply the 2 numbers on any output tube and you get the plate-watts-dissipation. Every tube has a maximum number for this that should not be exceded or the tube will "redplate" or "runaway" or worse shoot blue sparks inside. For example JJ 6L6 have a Max-pwd of 30watts. Say you have 450vdc on the plates.

30w/450v = 67ma

Now, you don't want to use this number because it is the maximum current that you'd allow through the tube. The general rule of thumb from my experience and what I've read is to go w/ 70% of the plate-watts-diss. So:

30w*70% = 21w

therefore at 450vdc

21w/450v = 47ma

hope this helps.

[ajeffcote]

I did have the red and brown OT wires switched.Reversed them and reset the bias.Tried it out and the screaming demon in the amp was gone.Still motorboating.Unhooked the NFB loop and that stopped.Maybe I need a larger NFB resistor?
I rewired the tone stacks.Bass pot 6.8K to ground and the .047 cap from the left lug(looking from the rear),wiper to the .1 cap and jumped to the treble left lug,treble wiper to volume hot,treble hot to 250p cap,volume left lug to ground and the wiper out to grid.Compnents are mounted to the pots to eliminate extra grid wire.The mid and bass cap together,to the 100K res,to the 250P and out to pin 7.The bass pots work,but there is not as much attenuation as I expect there should be.The treble pots don't change anything,or change very little.I must confess that the tone I am getting is just about where I would set the controls anyway,for the majority of my playing.But sometimes I want more treble and it sounds the same at 1 or 10.
No effect.
After all that said,the amp sounds fantastic at lower volumes.I can crank it this weekend and give it a good workout,but it is a different amp now.It's not a DRRI anymore.It sounds like the amps I played on in the 60's!
As far as the NFB loop goes,do I even need it?Is that just to level out the frequency response?

[ajeffcote]

This morning I replaced the tubes with all new or almost new tubes,including an EH 5AR4 and Tung-Sol 6V6GT's.I am getting more treble now,but the treble pots still .........do nothing.The bias tremelo is killer!

[lowell]

typically nfb is unnecessary IMO. If you want a cleaner amp i.e. more headroom it helps with that... it can also help w/ oscillations and stability but I've never had a problem when removing it and always like the tone WAY better as it definitely kills some "lively" response that the amp may otherwise have as well as dampens tone and makes an amp sound "dead." My opinions anyway.

[ajeffcote]

Since it is canceling some of the signal,wouldn't that also lower the gain?I like the way my amp sounds now.Very lively.I think I'll leave the NFB out.
I still can't figure out why the treble pots aren't working.I have wired just as the AB763schematic suggests.