Too high plate voltage on preamp tube

[slidincharlie (Carlo P)]

(I posted this question in my thread on "strange non-stock resistor" below. No answer. I try again here... TIA

I've measured the plate voltage on the reverb recovery tube (V4) in my SF Deluxe Reverb:
- 237 and 241 VDC instead of 170-180 on the schematic!

Should I correct the high plate voltage on V4?
If so, how?

Many thanks in advance.

[Rick Erickson]

[QUOTE=I've measured the plate voltage on the reverb recovery tube (V4) in my SF Deluxe Reverb:
- 237 and 241 VDC instead of 170-180 on the schematic! [/QUOTE]

I assume the voltages at the other preamp tubes are close to the schematic? This means that the tube in question isn't pulling enough current through its it's plate resistor(s). Since both sides of this tube share a common 820 ohm cathode resistor I would start by checking its value & make sure it has good continuity to ground. Also check the plate resistors themselves and make sure the grids have a DC return path (resistance) to ground so that they are being held near 0VDC at idle. If all this looks good it might be a bad tube.

***Note - All the usual precautions apply here - if you aren't familiar with tube amp servicing then you should take it to a professional. There are dangerous voltages inside your amp that can hurt or even kill you. Nuff said?

RE

[slidincharlie (Carlo P)]

Higher plate voltage on ALL preamp tubes

Rick,
I checked all other preamp tubes: the plate voltage in ALL preamp tubes (except the reverb driver) are 30-35% higher than stock.
I did the following Dan Torres' mod to have reverb and tremolo also on the Normal Channel (http://www.blueguitar.org/new/schem/...both_rev.gif): can this mod be responsible for the higher voltages?

[Tom Phillips]

Carlo,
If all the pre-amp tube voltages are high the cause may be one of the following or a combination of several of them.
5AR4 / GZ-34 installed when original SF rectifier was 5U4
High AC voltage from PT secondary
AC line voltage high
Power supply series isolation resistor values low
Pre-amp section drawing lower than normal current
Low idle bias current

The Torres mod should not cause the high voltage. (Other than the general bad karma of having anything associated with Torres in your amp )

Regards,
Tom

[Bruce / Mission Amps]

Here is a very simple 15 min reverb and vibrato on both channel mods I do in my shop for $15.00.

[slidincharlie (Carlo P)]

Tom, here's an answer to all your questions:
- a Sovtek 5U4GB is in the recto socket (I've put a NOS Philips there to no avail);
- the PT secondary gives 351 VAC (the schematic calls for 330 VAC);
- AC line voltage is a fine 118 VAC (comes from a stepdown auto-transformer);
- the 2k2 and 10k resistors between the filter caps read ok; voltages at the PS nodes are 426, 423, 401 and 339 (after the 10k) VDC;
- how do I check if the whole preamp section is drawing lower than normal current (and why should it behave so)?
BTW, this is the '77 pull-boost , original US (not the export version) model.
Re: Dan Torres: LOL !

[Tom Phillips]

Hi Carlo,

OK I think we are on to something here. But first, I should mention that there are lots of Fender amps out there with actual voltages that vary quite a bit from the schematic numbers. It’s usually not a problem and I think it’s due just a much to mistakes in the published service information as it it’s due to variations in the installed parts.

First your PT secondary winding is a little higher than nominal. I think this accounts for about 30V extra DCV in the pre-amp.
Next, your amp has the 2.2k and 10k values in the power supply. The original BF circuit used 10k and 10k so there was more voltage drop there. The newer pull-boost schematic still lists the same pre-amp tube plate voltages as were listed in the old AB763 BF schematic. This doesn’t make sense and it tells me that Fender never updated all of the information on the schematic as the circuit evolved. That may well account for the remaining difference in the actual vs. schematic voltages.

You may want to change the 2.2k to 10k and use the value that produces the sound you like best.

The higher PT voltage could be reduced using the Zener diode method if it bothers you. I’d just leave it as is though. It’s within expected tolerance.

Regards,
Tom

[slidincharlie (Carlo P)]

Thank you Tom. I'll change the 2k2 to 10k, and if it brings the voltages closer to expected, and I'll like how it sounds, I'll leave it that way.
BTW: why did Fender change that resistor? Just to have higher voltages in the preamp and a little more output power (or gain)?

[Tom Phillips]

...BTW: why did Fender change that resistor? Just to have higher voltages in the preamp and a little more output power (or gain)?

Never really gave it much thought. I just wrote it off as a Silver Face desighn change thing. I suspect that the CBS engineers were trying to get a little more headroom.

[slidincharlie (Carlo P)]

I've replaced the 2k2 resistor in the PS with a 10k, and this brought the plate voltages in the preamp down by about 20-30 VDC. I think this is enough.
According to the schematics, BF amps had 27k and 10k in the PS, but I kind of like the extra headroom allowed by smaller resistors.
Thanks,
Carlo

[Rick Erickson]

According to the schematics, BF amps had 27k and 10k in the PS,
Carlo

The non-reverb BF "Deluxe Amp" used the 27K. The reverb amps used 10K. You have a Deluxe Reverb, right?

RE

[slidincharlie (Carlo P)]

Yes, correct.
Sorry.
Now my SFDR has got two 10k resistors in the power supply.

[wizard333]

Hey Bruce..

Bruce looking at the mod you do to get effects on both channels, the only difference between your mod and the Torres mod is that you end up using the same coupling cap for both channels pre-verb, while his uses the existing separate coupling caps for each channel, which are often different values for each channel.

Since coupling cap values affect tone, I'd think that using one for both and raising the value (typically .022uf on the 'effects' channel, .047 on the non effects channel, or for vibrato/non verb amps .047 and .1) will change the sound of that channel and remove some of the differences in the sound between them, a plus or minus depending on what you are going for.

Unless your goal is to make both channels sound more similar to each other, I dont see the advantage. Either mod will put both channels in phase with each other in a verb amp, and either way they could couple through the common cathode circuit of stage 2 if you are "Y-cording" them unless you added another cathode cap/resistor to separate them.

Is there some advantage to doing it that way that I'm missing here?

More on topic, I've seen SRDRs with 10k and 2.2k in the filters, and all sorts of odd values. I generally change them to 10k and 10k because people want the 'blackface' apropriate values.

Plate voltages well above schematic seem common on SFDRs, I have one that runs at 460v. Thats *WAY* above the maximum printed value of 350v in the RCA book, but even the Fender schematic value of 415v is well in excess. Worrysome on those 6V6s but JJs seem to hold up ok. I've seen people stick 6L6s in them for that reason, but I'd be worried about the mismatch in impedance; the primary on those OTs is 8k if I'm not mistaken, well above the value appropriate for a 2x6L6 setup which IIRC want to see more of a 4k-6k primary impedance range. Maybe someone who has done it can elaborate on the appropriateness of that, seems like a big mismatch to me.

On the pre-amp side, as long as you arent exceeding limits in a big way, higher voltages will give you more clean headroom, a plus or minus depending on how you use the amp.

[slidincharlie (Carlo P)]

wizard,
your post raises a couple interesting points (sorry if my questions are very basic, I'm not a tech).

(...)Since coupling cap values affect tone, I'd think that using one for both and raising the value (typically .022uf on the 'effects' channel, .047 on the non effects channel, or for vibrato/non verb amps .047 and .1) will change the sound of that channel and remove some of the differences in the sound between them, a plus or minus depending on what you are going for.

Independently from the reverb issue, I have voiced the Normal channel in this SFDR (as well as in my SFVR) like a 5F6A Bassman. Would the Bruce mod affect this configuration? I do NOT want that both channels sound the same.

(...)I've seen people stick 6L6s in them for that reason, but I'd be worried about the mismatch in impedance; the primary on those OTs is 8k if I'm not mistaken, well above the value appropriate for a 2x6L6 setup which IIRC want to see more of a 4k-6k primary impedance range. Maybe someone who has done it can elaborate on the appropriateness of that, seems like a big mismatch to me.

Many people "upgrade" their BF or SF Deluxe with a ss rectifier and a pair of properly biased 6L6. I have just bought a Weber copper cap (the type with no sag) to try the same in my SFDR. Is everybody out there (including me) making a big mistake?

[MWJB]

The impedance mismatch for a pair of 6L6 in a DR is not 100%, the OTs as far as I can remember are 7-7.5K. I also have a vague memory that the Sovtek 5881 is a better impedance match than other 6L6 type tubes, despite getting a hard time in reviews they don't sound bad in this amp.

Either way, fitting 6L6s to a DR is only a mistake if you don't like the sound.