Fender Dlx Rev build- last Q b4 fire-up.

[Tom Phillips]

...In the meantime, I asked my (hammond 291 BEX) xfmr supplier [hugely knowledgeable/ decades of gtr amp tech] about the high voltages with the GZ34S. He said no need to get another rectifier/ I can still use the GZ34S fine: just set the bias pot to ~ -33v with amp in standby mode (wait/ not flipped UP yet) reading @ pin 5 of the 6v6's.. then measure the V @ pins 3 of the 6V6's with standby off (amp fully on/ flipped UP).. and I should read a useable V. ...

That is an incomplete description of the procedure for setting the bias of a tube power amp. Either:
1) You misunderstood the explanation or
2) That tech is lacking.

When you set bias you are adjusting the bias voltage to achieve a plate current that results in a plate idle dissipation power that falls within a specific range for the given tube. Missing from the description is the bias current measurement. For your circuit the plate dissipation is calculated by multiplying the measured plate current by the plate voltage after you have set the bias voltage. For the 6V6 the maximum allowed plate dissipation is 14 Watts. I shoot for around 9 Watts for the 6V6. That value sounds good and I don’t find it necessary to always set for 70% of maximum dissipation which is the often quoted internet lore value.

It is true that a higher bias current will load down the plate voltage but you can only take that so far. It is a BAD idea to increase the bias to more than 70% of max just to reduce the plate voltage.

...But the range of the pot goes from -60v to only -42v max turn one way/ other way...

That range is out of the norm but still might work fine with you high plate voltage. Is the circuit stock? If you can’t achieve the proper bias current with that adjustment range then we can modify the bias supply circuit.

[Sea Chief]

That is an incomplete description of the procedure for setting the bias of a tube power amp. Either:
1) You misunderstood the explanation or
2) That tech is lacking.

When you set bias you are adjusting the bias voltage to achieve a plate current that results in a plate idle dissipation power that falls within a specific range for the given tube. Missing from the description is the bias current measurement. For your circuit the plate dissipation is calculated by multiplying the measured plate current by the plate voltage after you have set the bias voltage. For the 6V6 the maximum allowed plate dissipation is 14 Watts. I shoot for around 9 Watts for the 6V6. That value sounds good and I don’t find it necessary to always set for 70% of maximum dissipation which is the often quoted internet lore value.

It is true that a higher bias current will load down the plate voltage but you can only take that so far. It is a BAD idea to increase the bias to more than 70% of max just to reduce the plate voltage.


That range is out of the norm but still might work fine with you high plate voltage. Is the circuit stock? If you can’t achieve the proper bias current with that adjustment range then we can modify the bias supply circuit.

Tom,

the circuit is a DRRI fender Main board with fender filter caps board; fender rev xfmr part no. same as RI, accutronics rev tank same part no. as RI (just PT & OT not fender RI, being hammond & Mojotone respectively).

I think the tech was not suggesting setting the bias per se, but setting the bias pot to about -33v he said, to then see a lowering of the plate voltage to a useable figure. Actually I did see a lowering, albeit only 8v or so at max pot twds RHS (this was -42v): so it was 478v & is now 470v (with the 5AR4/ GZ34S).

Ok so I bought a 5U4GB (EH/ cheapy- sorry but each tube at £20 and this is getting a pricey exercise) and stuck that in expecting to see a nice (relatively so) 430v or so. Nope. Pin 3 of the 6v6, even with the bias pot dialed slightly in favour of its RHS (-47v).. I got still a high 452v.

So Im I right: assuming I can't get my plate voltages any lower than this bar a few v.. I'm not too pleased this PT is labelled Deluxe Reverb.

Also the reverb doesn't work (just audible but poor quality, at max knob 10)- but I will come to that another thread.

[Tom Phillips]

Sea Chief,
A critical piece of information that is still missing is the plate current measurement. The voltage across the 1 Ohm cathode resistors you installed (or plan to install) will allow us to calculate the plate current. We need to know the idle condition plate current for each 6V6 AND the plate voltage under the same conditions. Can you supply that information. Without that information we are flying blind.
Cheers,
Tom

[Sea Chief]

Ok Tom will do. It will be midweek before I can get some 1ohm R's.

In meantime can I sub in a 12ax7 in the V3 position (12AT7 reverb driver) to tell if my rev prob is the tube?

Fwiw I gave the DR its 1st run today (once the 5U4GB in) and apart form the reverb prob, it works pretty well: the tremolo is good (just a tiny bit of "motorboating" but can easily live with), the amps quiet, doesnt get hot just barely warm on cab top which I like. Sound is a bit 'flat' but otherwise good. A little noisy/ buzzy when I don't touch the strings, but iirc not unusual for gtr amp- be good to get the buzz awol tho.

[Sea Chief]

Those testers are for efficiency. If you do repairs they allow you to simply plug them into a customers amp rather than make circuit modifications before being able to run tests. If you're just a player they allow you to monitor bias without the need to order parts and take to a soldering iron. And there may also be times when a repair doesn't even require removing the chassis. In this case, if the bias has no need of adjustment you save that trouble because it can be tested outside the amp.

I don't own them

Aha I get it thanks- taking the amp out is a pita every time for sure.

many thanks chaps- SC.

[Tom Phillips]

...In meantime can I sub in a 12ax7 in the V3 position (12AT7 reverb driver) to tell if my rev prob is the tube? ...

Sure. That won't damage anything.

...A little noisy/ buzzy when I don't touch the strings, but iirc not unusual for gtr amp- be good to get the buzz awol tho.

Sounds normal. Especially if you are using single coil pickups. Noise that goes away when you touch the strings or when you turn down the guitar's volume control is being picked up by the guitar. There is nothing you can change within the amp to stop those particular noises.

[Chuck H]

Ok Tom will do. It will be midweek before I can get some 1ohm R's.

In meantime can I sub in a 12ax7 in the V3 position (12AT7 reverb driver) to tell if my rev prob is the tube?

Fwiw I gave the DR its 1st run today (once the 5U4GB in) and apart form the reverb prob, it works pretty well: the tremolo is good (just a tiny bit of "motorboating" but can easily live with), the amps quiet, doesnt get hot just barely warm on cab top which I like. Sound is a bit 'flat' but otherwise good. A little noisy/ buzzy when I don't touch the strings, but iirc not unusual for gtr amp- be good to get the buzz awol tho.

Don't sub a 12ay7 for the 12at7. That is being used as a power tube and the 12ay7 can't handle the work as well. Ok... Maybe for a second, but don't leave it in there.

If the amp sounds "flat" you could try reducing the NFB. It's not unusual for BF Fenders to have what most players consider a more workman like/average clean sound. Does your design include the 47pf cap permanently across the volume control for the "bright" channel? DR's do sound best when they're pushed, but I wouldn't suggest that before knowing you're bias circumstances. For a little more lively tone at moderate levels a reduction in NFB can help. A popular mod is to add a variable NFB control. The reverb may help too once it's working.

The buzz when you take your hand off the guitar probably won't be something you can fix with the amp circuitry. Any amp set up with the same gain and volume should do the same with that guitar. If not, it's possible the DR has DC on the input jack. Easy to test for. Just unplug the cord from your guitar and place your meter (set to read DCV) across the terminals.

[Sea Chief]

Well well some assh*le left a piece of foam in the reverb tray over all the strings didnt they.. silly c**t whoever that was..

Dont think there is a 47pf cap on either channel- not on the schematic afaict. I will bear in mind the nfb as it perhaps sounds a bit flat still, not as 'sparkly' as I'd hoped Alas it wont be pushed- not possible at all in nr future/ neighbours etc.

Sounds great now reverbs sorted tho! defo a great synergy with my gibson sg/ humBs.. even with a crap spkr and these cheap 6v6's & rectifier its thick and pure, and the reverb is glorious! (but a bad hum esp above 5). The main thing tho- it sounds like a fender DR and -not- as I think so many builds sound, like a 'kit'/ homebuilt DR even if ptp.. which is exactly what I was hoping.

Show me how to put a pic up? Id like to show my build. Terrific help chaps! Im happy now. Just the R's to add and current reading/ tweaking & all's done really.

Thanks SC.

[Chuck H]

There should DEFINITELY be a 47pf cap across the two ungrounded lugs of the volume control on the vibrato channel. That's sort of the DR signature difference! Of course a 50pf (or 51pf or 45pf, etc.) will do if that's all you have BUT GET ONE ON THERE! That and the bias may need final adjusting. Then see what you think.

[Tom Phillips]

...Show me how to put a pic up? Id like to show my build...

There are instructions under "Attachments and Images" in the forum FAQ at Reading and Posting Messages

[Sea Chief]

There may be one actually- can you have a look on the schematic (link #1).. C18? Inputs Top LHS.

[g1]

You mean C10?

[Chuck H]

There may be one actually- can you have a look on the schematic (link #1).. C18? Inputs Top LHS.

That link is only for the footswitch. Looking at the "65 Deluxe Reverb" (reissue model) schematic it would be C10. C18 looks to be part of the trem circuit.

[Sea Chief]

That link is only for the footswitch. Looking at the "65 Deluxe Reverb" (reissue model) schematic it would be C10. C18 looks to be part of the trem circuit.

Yes I meant C10 (G1).. tiny 18's all look like 10's/ vice versa on my A4 print.

[Sea Chief]

ChuckH- would a 1/2W 10r do? my store dont do 1W r's.. and I got plenty of 1/2w but smallest 10r (s'one said they use 10r for this addition anyway).

Thanks, Sea Chief