59 Bassman Screen-Grid Resistor Burn

[Bourgeois]

Dear Forum,

I am a guitar player from Istanbul, Turkey and I am just registered to the forum in search of advice/recommendations of you knowledgeable and experience members. I would appreciate your time if you could assist me solving my problem - since I am far away from skilled amp builders and parts.

I do play a reissue 59 Bassman LTD (2009) European 230V version. Following the info and recommendations on the internet I have swapped V1 with an 12AY7 and power valves with Phillips 6L6GCB after I purchased it at that time. I have some technical skills I can work on amps and bias my amps based on information I find on the internet.

Last week I returned home and a layer of smoke and burnt electronic smell welcomed me. I usually play guitar around 3 to 4 hours an evening; sometimes I forget to turn it off before I go to bed. I guess this was the case. I have opened the back cover and clearly seen that the circuit board has burnt just behind R32. I have checked the valves - and they were also blackened and the 1.6A fuse was also burnt. I guess the old valves shorted and burnt the screen grid resistor (thankfully they were fire proof ones).

I will unmount the circuit board and throughly check all the components. The fuse, the R32 resistor and other failing components will be mail-ordered from US or Europe; but I would like to get your experienced amp professional's opinions on what extra parts I can change - once the board is out - to add more reliability and tone to the amp?

I have budget to order NOS parts (resistors, caps, etc...) and some stock from a Marshall re-build I may use on this one. I would love to drop the B+ on this one to around 400V to be able to use 5881s w/o problems but I do lack knowledge on how to accomplish this.

Thank you very much for your help in advance...

Best Regards

[beasleybodyshop]

Can we see some photos of the chassis?

[Bourgeois]

Hi,

You may find attached the images of the amp, board and valves I took out.

I have cleaned the circuit board surface with isoprophanol, but the remaining black spot is the board roasted - the spot will stay. I hope it didn't harmed the board and I can still use it. In the worst case I can replace it with an eyelet board.

[Randall]

Looks to me like one of the 6V6s shorted and burned up it's 470 ohm 1 watt screen resistor. This will not be an easy repair.

RESEARCH ABOUT HOW TO SAFELY WORK INSIDE AN AMP AND HOW TO DISCHARGE POWER SUPPLY CAPACITORS.

I would first snip out both 470 ohm resistors as well as the 1 ohm bias check resistor on the right, before I removed the board. All the black carbonized circuit board must be removed before the components can be replaced, because the carbonized material can become conductive and cause more trouble. You can do this with a small Dremel tool, or whatever means you have.

Once the board is scraped clean and wiped with alcohol, then you can set about removing the board. Write everything down, make a map of all the wires you disconnect. Once the board is flipped over, then the burnt traces, if any must be repaired. Same thing as before if there is any black burnt material.

Once you are satisfied everything is good and solid, you can replace the two 470 ohm 1 watt and 1 ohm 1 watt resistors. Reassemble. Install new tubes. Test. Don't get electrocuted.

[Milkmansound]

good opportunity to gut it and put a hand wired board in there

that is only a *little bit* more work than replacing the burned screen resistors

[billyz]

I usually move the screen resistors to the tube socket when the board has charred like this. Clean the burned area well and measure for any continuity between traces etc. 5881's can handle 450vdc no problem , fender did it all the time. New 5881 tubes are not really 5881's , just 6l6's . Easiest way to drop voltage is a Zener diode string on the secondary ground leg. Build a tag board cuz they are going to get real hot!

[Bourgeois]

Thank you very much for the assistance and advise, Randall, Milkmansound and Billyz...

I am confident with my basic amp knowledge and craftsmanship; I can do a tag-board build for this one but that would cost to forum time to answer hundreds of questions.

Another limitation is the fact that I am located in Istanbul, Turkey. Any part I would order online will be delivered in 2 weeks earliest - and we have limitations on the customs.

Should I go for a re-built, would it be 5F6-A to go for?

[10thTx]

You can purchase an assembled board from Ceriatone in Maylasia:

http://www.ceriatone.com/productSubPage ... tParts.htm

So that might be an option for you?

With respect, 10thtx

[vibratoking]

I didn't hear you complaining about the amp's tone when it worked properly. Based on that, I would suggest you perform the repair as some others have said. You can fix this without a major rebuild and it will be as good as it was. You'll spend a few hours instead of a few days. I'd get back to playing if it were me.

[Bourgeois]

Dear Forum,

As you know I had my scree-grid resistors burnt leaving a carbonized spot on its PCB in my 59 Bassman Reissue LTD.

I will be travelling to US soon; and if you encourage me it seems I am pretty convinced to go for a complete overhaul to take it to vintage specs. If you also agree, I would like to buy the heavy parts and bring them back with me - and for the rest I can always mail-order them. My shopping list is as below:

- MM PT to bring B+ down to 400V (with 230V European Power)
- MM OT (which version should I use, they have many?)
- Weber Speakers - 10A125 (I will only be able to carry 2 of them with me; I will continue to use existing 2 in mix) WHich version should I go for?
- Turret Board with some parts

Since I dont live in US, I dont have the option to sell it and buy a vicky or find a vintage one. I love my amp and I would love to see it once again alive even better than before.

Many thanks in advance for your comments, feedback, answers to my questions...

BR

[Bourgeois]

Dear Forum,

I understand my above plan is so silly that I could not get any feedback.

Considering that I live continents away from US and not having much amp experts around I rely on the information/guidance and advices from you during this task. Thank you very much for the time you spend on this thread, once again.

I am in the process of choosing a PT for my amp from Mercury Magnetics catalog. There are many models supporting our 230V. From what I have read so far in forums, in order to be able to use 5881s in the amp we must reduce the B+. From my notes from latest valve swap and BIAS; I see I have noted B+ as 443V. In all forums people say that the RI Bassmans put around 460V on the plates.

Have you had any previous experiences with Mercury Magnetics PT plate voltages? Which model should I go with in order to be able to use 5881 valves?

And if any of you have worked with export versions of the amp; there are too many cables going out the PT as opposed to local versions to the PCB. Do you know why is this difference?

Thanks again

BR

[statorvane]

If you are using the Sovtek 5881s I wouldn't worry about plate voltage that much. Those aren't really 5881s, but they are extremely robust. They are the King Tigers of vacuum tubes. Others, I'm not so sure.

I ran them in a homebrew 5F6-A at around 435 volts on the plates. Never had a problem. Later re-tubed and rebiased w/ JJ 6V6S. Same deal, not problem. This was with a Weber transformer pair.

[JazzGuitarGimp]

For the transformers, I would suggest you either call or email Patrick at MM. Tell him what amp you've got, and what you want to rebuild it as. Tell him what plate voltage you are shooting for, and whether you're using silicon diodes or vacuum tubes for the rectifier. Then ask for his recommendations as to the PT and OT that will work best for you. Patrick is very helpful and easy to work with.

Here is his contact info:

Patrick Selfridge
email: Patrick@MercuryMagnetics.com
Telephone: +1 (818) 998-7791

[ampgeek]

I fully concur with Statorvane! I regularly used quads of Sovtek 5881 WXT in Ampeg V4s with absolutely no problems. Plates run at over 525 VDC IIRC.

In fact, at the risk of being called a heretic, I thought the 5881 WXT's sounded better than NOS RCA 5881's in almost every suitable amp that I tried them out in. YMMV of course.

So....if you are thinking about switching out your PT solely for the purpose of lowering power tube plate voltage to reach vintage sonic nirvana....I would first try some of the Sovtek offerings to see if they suit your fancy. You might save a whole bunch of money and time.

At worst case, you will end up with a very decent set of spare output tubes that you can use in a pinch when needed!

Good luck,
Dave O.

[Structo]

When you replace those screen resistors, mount them up off the board about 1/4" so they can dissipate the heat better.