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)]

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]

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.

[wizard333]

Depends on how exactly you are making that conversion and where you are puting things. The difference between the channels on a 5f6a amounts to one .0001uf coupling cap on the bright channel. Each channel uses half a triode and they share two halves of a triode after joining. I'd have to see a schematic of how you are wiring things up to tell you how what was posted would affect things. What you want to do is make sure that .0001 cap on your bright channel occurs before you combine the channels; i.e. before the wire that is added in the mission mod or moved in the Torres mod.

As for using a solid state rectifier, a few things are going to happen. Your b+ will go up all through the amp. Since 6v6s are already over spec in a DR, I'd definitely NOT do that with 6v6s, it may be ok with 6L6s but then you are hitting that impedance mismatch in the primary of the OT again...... Also, your b+ will come on FAST and spike when it comes on, which can be a problem if you have any weak components, its likely to stress them and may cause failure, maybe catastrophic failure. This is especially true on a DR if you are using the stock filter cap configuration (even with brand new caps) because they wire 2 16uf caps in PARALLEL, which gives you 32uf BUT DOESNT double the voltage handling capacity. If you are using 500v caps, and even moreso with lower voltage caps, this can definitely over stress the caps and cause catastrophic failure, taking tubes and output transformer along for the ride. If you MUST install a solid state recto in a DR, change the wiring on the first two filter caps to the standard higher wattage two-caps-in-series with 220k resistors across both to even up the voltage handling. If you use two 80uf or 100uf caps, this gives you 40uf or 50uf but the voltage handling of both caps adds, so you would get 800v handling capacity out of two 400v caps, which should be more than enough.

A GZ34 ramps up slow, which protects components, and many prefer the sound (softer, more 'bloom') of the tube rectifier. If you change your filter cap circuit to two 100uf caps in series, you can still switch back to the GZ34 (you'll have to rebias). You want to be careful how much capacitance you have in the first section after a tube rectifier, but 50uf is fine for a GZ34 (though it might be too much for other rectifier tubes).

Overall, my recommedation would be to NOT put a solid state recto in a DR, but if you just have to have one, you should definitely do the change I mentioned above or you are asking for trouble.

[slidincharlie (Carlo P)]

Thanks wizard,
I have often read about this "ss recto + 6L6 mod" in DR's, but this is the first time that I read such caveats.

One more (last?) question:
since I'll want to try ss recto + 6L6 a little once in a while, what should I do when revert back to stock 5U4GB + 6V6's?
May I leave the first two filters in series, or should I rewire them in parallel?

[wizard333]

If you wire them in series, leave them that way its better all the way around. Thats assuming you replace the little stock 16uf caps with 80uf or 100uf caps. Dont even try parallel with the stock caps.

The stock rectifier tube is a GZ34 I'd use that. If you use another rectifier tube, it may 1) Not be able to handle the first stage capacitance 2) Draw too much current from the heater circuit (5U4 draws more current than a GZ34, your transformer may not like that, it also cant handle as high of a first stage capacitance as a GZ34) 3) ramp up differently than the GZ34.

Rectifier tubes may or may NOT be interchangable depending on the amp (first stage capacitance, whether the heater can supply the current required). A GZ34 tube is a good tube for that amp, stick with it. If for whatever reason you want to lower the B+, do it with a bigger resistor.

Again, I'd recommend against a solid state rectifier in that amp. People may do it, but some people also can't seem to live if they dont overclock the crap out of their computers, which makes them unstable and can cause major issues, including catastrophic failure that takes expensive components to the graveyard. I much prefer stability and maintenance free performance to a money pit experiment.

[slidincharlie (Carlo P)]

wizard,
the stock rectifier in my pull-boost SF Deluxe Reverb is a 5U4GB.
Does this affect in any aspect your comment above?

[wizard333]

If its stock (i.e. the tube chart lists that) then use it, the tranny will be able to supply the required heater current. What year is it? Early 70's and before were GZ34s, not sure what year they changed.

Also in that case, I would go with 80uf caps in series in the first filter stage; 2x100uf may be a bit much for that rectifier tube.

[slidincharlie (Carlo P)]

It's a '77 SFDR. Currently it has a Sovtek recto, but I also have a couple NOS.
Thanks for all the advice.

[wizard333]

A rectifier tube is a rectifier tube, your signal doesnt pass through it I dont think I'd throw down for NoS but if you already have them by all means use them. I've heard Mullard rectifiers last basically forever, but I've never owned one.

Is your '77 an ultra-linear version? Not sure when/if they went that way with DRs but it would be interesting. What is the rated wattage listed on the back panel?

[slidincharlie (Carlo P)]

SF Deluxe Reverbs did not went ultralinear. The rated wattage is 20W (but I believe it is 22W actually).