Modified Twin Reverb Problem

[dpm309]

I just finished putting together a Silverface Twin that has been modified.  The owner brought this work in progress to me (he recently transfered to Arizona from California) that was half finished by a tech in California.  I installed new Mercury transformers, choke, power tubes, and a power supply board.  Checked voltages with the tubes removed and got high voltage on the plates of preamp and power tubes.  Installed the tubes and fired it up with it plugged into a light limiter with no issues.  I then fired it up without the light limiter and turned it on with no problem until I turned on the standby switch where I noticed crackling and smoke coming from the 1K resistor that connects to the bias tap on the PT.  I have attached a picture of the rectifier board.  I am still getting a reading of about 1K on this resistor but the markings on it indicate that it should be 1.9K.  With the standby switch off, I am getting about -40V which matches the schematic.  Should I be looking at the power supply hook ups or is there something else I might be missing?

Thanks,

Dan

[VMS]

What is the purpose of those resistors parallel with rectifier diodes?

[dpm309]

I have no idea what they are for.  When I received this amp, this board was already installed along with a new turret board to replace the fiber boards.  From what I can tell, the original circuit looked closest to an AA270   I will contact the owner to get some additional information on what kind of mods the previous tech performed.  I went ahead and replaced the 1K resistor with a 2W metal film and when I turned the amp back on, it did not smoke or crackle but I heard noises an immediately turned it off.  With the light limiter, I was able to determine that there is now a short of some kind in the power amp section.  When I pulled all of the power tubes, the limiter did not indicate a short.  In the meantime, I will check and recheck everything in the power amp and power section.

Dan

[worth]

This is what the "tech" was trying to do..

[eleventeen]

An idea or two you might try:

1: Obviously, there is some possibility that you have an internal short inside one of your 6L6 tubes.

2: With power off, try taking one lead of your ohmmeter on the wiper of the bias pot [edit: and the other lead to ground] and put the 6L6 tubes in, one by one. See if you get any big jump. You shouldn't.

3: Normally, the bias circuit uses miniscule current, I mean, tiny.

4: I have to suspect, at some level, those resistors across the B+ diodes. I have seen such resistors used in other circuitry (not in Fender amps) as so-called "equalizing" resistors, and whether they do anything useful or not, I don't know. The reason I suspect them in this instance is not that they would do anything in particular to the B+ circuit or the operation of the diodes therein. But we must not forget that the bias winding is a section or segment of the B+ winding...it's not a fully independent winding with its own + and -.....it shares the center tap on the B+ winding.  

[dpm309]

Worth - thanks for the information.  I suspected that they were there for additional protection. 
Eleventeen - The tubes are brand new but I will still check them as you suggested.

Thanks,

Dan

[eleventeen]

Also, I did not see any caps spanning the diodes on that "ultra rectifier" board, but if there are any and they are leaky, that would drive the bias circuitry into a parallel universe.

[stratele52]

dpm 309 it is a real mess this part of the amp . You have everything to check from A to Z with the proper schematic.

Another fake tech do that......

[stingray_65]

Not to hijack this thread,but...

I've seen in a few schematics that string of diodes doing the rectification, In a few cases I've blindly build them.

I've always wanted to know  WHY?

Will not a single diode work?

Ray

[eleventeen]

I think PRR explained the reason for the multiple diodes somewhere...but danged if I know where.

I think it's more than anything a legacy from a time when the PIV and PRV ratings for diodes weren't quite as well trusted as they are now.

Of course, you should ask "does it matter that the middle diode sees a measly .7 volt PIV and PRV less than it would if the first diode wasn't there?" No. "Since the diodes are in series the same current flows through all of them, so why does that matter?" Same answer.

[tubeswell]

Not to hijack this thread,but...

I've seen in a few schematics that string of diodes doing the rectification, In a few cases I've blindly build them.

I've always wanted to know  WHY?

Will not a single diode work?

Ray

See the paragaph about minimum ratings. http://www.freewebs.com/valvewizard/fullwave.html

[dpm309]

If you think the rectifier board is a mess, you should see rest of it.  Half of the components were not even soldered to the turrets and I had to also refloat the majority of the other soldered connections.  This amp barely resembles the AA270 schematic that it was before the mods.  Still trying to get some information from the owner as to what was done to this amp!  Note to self: - never take on a half finished project that another tech started.  Anyway, I am now suspecting the bias circuit which was also modified.  There are no caps spanning the diodes on the rectifier board.  The filter cap on the bias circuit is a 220uf/100V instead of a 50uf/70V as shown on the AA270 schematic but I don't think this is causing the problem. 

Dan

[eleventeen]

It wasn't clear from your earlier posts that the overall build quality was Slopville; the rectifier board a looked a little "ehhh" but...whatever. The high voltage rating on the bias cap means nothing. Depending upon how bad it is, this is probably another case for the red pencil (not felt tip pen) and the schematic, tracing things out wire-by-wire.

Yes, a part-finished project can be a nightmare. As I wrote in another very recent post, you don't have a "broken amp" there...you know, that was working at one time and then blew up or cooked something. You really don't know if what you have EVER worked. Those can be tough, you really have to check each and every solder joint, more subtly, have to confirm that every tube-pin connection has been properly made. There really is no lower bound on how stupid the mistakes you might find could be, but essentially nothing can be taken for granted. 

The only shortcuts I know of are to measure plate-to-plate ohms on the two separate halves of the 12AX7s; all but two tubes should read 200K, the series resistance of two plate load resistors; and, most of the 12AX7 cathodes should read 1K-2K ohms to ground. All the grounds should interconnect; the several B+ nodes should differ (in ohms) by the values of the 1-watt dropping resistors. (4.7K, 10K, 1K, whatever they are) Once those few things are under control, then it's just grind it out, wire by wire, part by part, with special attention to under-board wires. And you HAVE to use the red-pencil-on-schematic method or something very similar, because the task is so long it's VERY unlikely you'll be able to finish it all at one sitting. And it will STILL drive you nuts. 

[HotBluePlates]

What is the purpose of those resistors parallel with rectifier diodes?

Blind copying.

If you notice from Merlin's page, it is more common to see smallish caps spanning the rectifiers. IF you have to use multiple diodes in series, the caps will force voltage-sharing among each of the diodes, in the same manner that bleeder resistors can force voltage-sharing between series caps. Often, you might see series caps used as a voltage divider when resistors don't make sense to use. Opposite what you might think (unless you check the formula for capacitive reactance), if 2 different-valued caps are in series, the smaller cap has a larger voltage across it than the larger cap.

Anyway, it's all moot. Vintage Twin Reverbs used six diodes because the individual diodes did not have the Peak Inverse Voltage rating of modern 1N4007's. These days, you don't need 3 diodes in each leg; if the peak of the recitifed incoming a.c. does not exceed 500v, a single 1N4007 will suffice. People often use the 1N5408 for some psychological comfort, because it is physically bigger. In reality, it carries the same 1000v PIV rating the 1N4007 does, but can pass 3x the current.

The bias board pic looks like there might be a lot of solder/rosin splatter on the board itself, and the lug holding the bias tap wire looks like solder could be missing or have flowed down the lug and out the bottom. And is that some kind of zener diode in series with the center-tap? All-in-all, I'd suspect everything, and undo some of the "improvements."

If you think the improvements might help, leave extra space to reinstall them, but remove them and get the thing running stock first. Afterwards, you can add back those upgrades you think will yeild some benefit (and actually be able to compare them to the stock performance).

[Danskman]

This "build" seems to be a real mess... You'd better start over and build a fresh AB763 Twin, either with a Hoffman Board (which will give you very fine result and nice bias tremolo), or fiber board (if you want to add some vintage appearance to it). Trying to correct multiple failures or errors in a circuit that never worked is a complete waiste of time, IMHO. It can even last more time and you will be  at the end 

Good luck and best regards,
Danskman

[dpm309]

I would have preferred to build an install a new Hoffman AB763 board but the previous tech did construct and install a new epoxy resin turret board but it does not resemble the Hoffman board.  My job is to get it working.  Another strange modification that they did was to connect all of the power tubes' pin 8 (cathodes) to one 10 ohm, 10 watt resistor that ran to the ground.  The owner explained that this allows one to use mismatched power tubes and measure the bias across this 10 ohm resistor.  They also replaced the 10kl bias pot with from what I can tell is a 15k, 20 turn precision pot.  This supposedly gives more control over the biasing.  The amp came with 2 brand new matched pairs of JJ 6l6s.  Each pair has a different sets of numbers (17/2.6 and 12/2.2) so all 4 will not be matched. 

Has anyone ever heard of this biasing scheme?  How should I install these tubes to get the best performance given that they are not matched: one matched pair in V7 and V8, the other in V9 and 10 or one set in V7 and V10 and the other in V8 and 9 or does it even matter?

Thanks,

Dan

[eleventeen]

Yes, I have done that biasing scheme. It works OK. I don't see any big deal with replacing a 10K pot with a 15K pot. As for how to install the 6L6 tubes, one hot one and one cold one per side (of the output transformer) In other words, each side of the OT should see the same thing. It does not matter which of the two parallel sockets you use; imagine yourself standing on the output tranny looking at the 6L6 tubes. Your left "hand" should see the plates of one cold and one hot tube. Your right hand should see the same.

[dpm309]

I got this back on the bench and went over it with a fine tooth comb and did not found anything out of the ordinary except that one of the shielded cables between V1, pin7 and the Volume control on channel 1 was shorting out at the pin.  I trimmed the shielding and put some heat shrink and the amp now works!  I set the bias to 40mA and the amp sounds great.

I want to again thank everyone who gave me advice and suggestions and as I mentioned before, I will never take on another half finished project.

Dan

[eleventeen]

Yaay! Good job!

It's OK to take on a half-finished if the price is right. Just realize that half finished is usually twice the work!

[Willabe]

Yaay! Good job!

        What  he said! You stuck with it, hats off to you sir.     

It's OK to take on a half-finished if the price is right. Just realize that half finished is usually twice the work!

Hmmm,        I like that. Good words.


             Brad