Nice mess in a 78 JMP Superlead...need advice!

[martin manning]

Here's my put at just fixing the bias with a doubler circuit:

First, as long as the underside of the board is accessible, I'd replace the two 10uF caps in the bias circuit, as they are probably past due.

Get rid of the piggy-backed resistor and the 15k resistor with the jumper.

Then, use the half-wave doubler circuit below.

With this you can leave the existing circuit as is, with three new parts as shown below (R1, C1, and D1, a 1N4007). R1 and C1 go in the place of the 15k, and Di goes from the junction of C1 and the existing bias diode to ground (ground it with the existing bias supply caps. In my diagram the 58k resistor is the sum of the 47k resistor next to the existing bias trimmer, plus half of the trimmer's resistance, representing mid-range.

[Cameron]

Here's my put at just fixing the bias with a doubler circuit:

First, as long as the underside of the board is accessible, I'd replace the two 10uF caps in the bias circuit, as they are probably past due.

Get rid of the piggy-backed resistor and the 15k resistor with the jumper.

Then, use the half-wave doubler circuit below.

With this you can leave the existing circuit as is, with three new parts as shown below (R1, C1, and D1, a 1N4007). R1 and C1 go in the place of the 15k, and Di goes from the junction of C1 and the existing bias diode to ground (ground it with the existing bias supply caps. In my diagram the 58k resistor is the sum of the 47k resistor next to the existing bias trimmer, plus half of the trimmer's resistance, representing mid-range.

Thats not a voltage doubler. Thats the b+ bias type circuit
The improved bias circuit on the bottom.

[Firestorm]

Martin is not wrong. Just sayin'.

[Cameron]

Martin is not wrong. Just sayin'.

looked to quick...you are right or Martin is right.

[donzoid]

I think even I can pull that off.

Correct me if wrong:

D2 is the existing diode.
The caps (including the two replacements) should what type (if it matters)?
The new diode -- to ground ... with the same point as the 10uF bias caps. Yes I see the direction of it is opposite (it even makes sense now).

I may be able to get the needed parts but its slim picken's for supply stores around here so will have to do it tomorrow. But thanks for spelling it out for me. Yes, I did need that. But now, it's achievable even for me.

[renshen1957]

???? WTF Marshall amps have a bias trim pot in the amp...not just a resistor. Boogie and Peavey and others have just a resistor.

Hi Cameron,

The resistor or pot in question relates to Don's deciding between switching 6550 or EL 34 tubes, the amp does have a bias pot even if the bias supply leaves something to be desired. My aside on why resistors are chosen over pots should have had a for example, thank you for pointing out the resulting unintended faulty reference.

Don's most recent observations on the bad soldering, with the majority of the faulty work located in the bias section is a good enough example why I rail against poorly designed and executed PCB boards (especially Fender's) mostly as the designs are full of comprises to begin with.

Marshall's first PCB at least kept the same component spacings and power traces separated from audio traces. Arguments for the PCB have been for less variation between units or improvement of consistency of sound quality, easier mass production in some cases. Ease of access or simplicity of repair isn't one of the arguments for pcbs as I have seen from Peavey, Fender, Marshall et all. By the late 1980's Marshall was into packing components into much smaller PCBs. However, I digress.

If this was 9 years ago the collector's value would have ranged between the price of the parts to twice this value depending on condition, mods, issues. The value has gone up, not to the stupid money levels of pre-PCB Marshalls but a check on ebay might or not surprise some.

However, ownership of the problem. Fixed Don might be able to build his dream amp if sold this amp. However with small children funds are tight to pay a tech. Or the amp of his dreams could wait as this amp would do in the interim, the amp garage is a great resource to assist in repairs This could then be the amp he could work on to learn about amp repair.

I've repaired PCB amps (tube and solid state guitar and hifi), and most commonly contained undersized (if the correct wattage) resistors, caps, etc which weren't selected for tonal considerations or durability, but just to fit the circuit board construction,and be as cheap as possible. Anyone with experience with radios or hi fi amps is keenly aware not the special requirements,

If I had a bench charge and charged by the hour, I'd love pcbs, they would put my kids through college.

Most musicians who scrape together enough dough don't walk into a chain guitar store and ask the clerk for a printed circuit board guitar amp. They hope they can afford a tube, and between cash flow and tone, they do the best they can.

If the amp was mine, no question, I would build something I wanted, but I wouldn't build a PCB board amp. So much easier to service PTP amp and I would have my choice of components adjust my build accordingly as well as

Best regards

Steve

[martin manning]

Yes D2 is the existing diode. Ground the new diode with the existing bias supply caps. The new caps should all be 10uF 100V.

[JazzGuitarGimp]

I think even I can pull that off.

Correct me if wrong:

D2 is the existing diode.
The caps (including the two replacements) should what type (if it matters)?
The new diode -- to ground ... with the same point as the 10uF bias caps. Yes I see the direction of it is opposite (it even makes sense now).

I may be able to get the needed parts but its slim picken's for supply stores around here so will have to do it tomorrow. But thanks for spelling it out for me. Yes, I did need that. But now, it's achievable even for me.

The caps should be aluminum electrolytic.

[Cameron]

???? WTF Marshall amps have a bias trim pot in the amp...not just a resistor. Boogie and Peavey and others have just a resistor.

Hi Cameron,

The resistor or pot in question relates to Don's deciding between switching 6550 or EL 34 tubes, the amp does have a bias pot even if the bias supply leaves something to be desired. My aside on why resistors are chosen over pots should have had a for example, thank you for pointing out the resulting unintended faulty reference.

Don's most recent observations on the bad soldering, with the majority of the faulty work located in the bias section is a good enough example why I rail against poorly designed and executed PCB boards (especially Fender's) mostly as the designs are full of comprises to begin with.


Steve

I would just use a switch but.....Don is new to this and why give him too much info just to confuse the issue. We are not sure if the solder is good or bad....He does not have much experience...so I would keep it simple and help him get the amp running right instead of bringing up more things to fuck with. After getting the amp running good then he can see whats next for it.
For me it only takes a small bit longer to fix or mod a PC board amp........fix.....not continuously fucking with it......so I don't charge more to fix it. If you wanted to learn to mod or fix then a PTP amp is the way to go. More repair guys make allot of money saying to there customers they need a new PTP board and all new parts... when they don't....mostly because the repair guy lacks experience.

[donzoid]

I will go on the parts search tomorrow. Hopefully I will find what I need. I think after all the folks who have helped on this thread, once this pup goes back together it will deserve a "fair quality" sound clip. I'm pretty stoked to be able to put this thing back together, and know that at least the things I have touched will be better than I found them...my dream amp? Probably a TW. I find this activity very advancing toward that. I sure wish TAG existed 20 years ago. But it sure is cool NOW. Thanks all!

[donzoid]

Thats a good idea too Cameron...but getting back to your deep breath comment, I agree I would like to hear the thing running right with the current set of tubes before mucking with an actual mod. The idea of a pot, was not to have to take out the chassis for something so basic. Since I have a bias probe, I can "see" what I have so it was for convenience more than anything. I use the term mod here as a change for different tone, and regard this as a repair or correction if that makes any sense.

[donzoid]

the 4k2, should I make that 1 watt or 2 watt?

[Structo]

I like to use 2W-3W metal oxide (flameproof) resistors in the power supply and bias supply.

[donzoid]

Thanks Tom!

[renshen1957]

More repair guys make allot of money saying to there customers they need a new PTP board and all new parts... when they don't....mostly because the repair guy lacks experience.

Hi Cameron,

That hasn't been my experience.

Some people know I build amps, ("Oh you do? I have this SS Harmon Kardon amp with the left channel not working," My Peavey Transistor amp just gave up the ghost...") through friends and acquaintances. But it's not my main business.

Of these people who have asked me to build a vintage, circuit, i.e., Blackface Deluxe, into another amp fall basically into several categories:

1) They purchased a Fender PCB reissue that didn't stack up to their expectations or an original amp tone or performance qualities. If I am busy (most of the time with my company), I tell them to sell the amp and go out and buy either a Silver Face or from a boutique builder such as Allen or Clark who build clones. There used to be a builder on Ebay who did this type of conversion who received glowing testimonials with the results from his clients.

2) The person got ripped off with a bargain to good to be true purchased off Craigslist, which they could barely afford, tube amp which afterwards required repairs . Most commonly these are Hot Rod or Blues edition amps that the heat from constant use lifted the traces, etc which worked long enough to make the sale. If am busy..., I suggest they get their money back by parting out the amp (especially the damn PCB I recommend saying its for the parts value only) on Ebay.

However, I remember the excitement of many a friend or family member's first tube amp, a used vintage amp, one they could barely afford. I am more inclined to take on this type of project as the person got swindled or had just plain bad luck, plus to some, this would be their first, last and only tube amp. Money is an issue for these people. My wife isn't happy as I barely break even covering the price of the parts and next to nothing for my time when I could be out making money instead.

3) "Peavey Classic 100 that eats through EL84s, catches on fire, or is always in the shop?" It isn't just Fender.

4) A Bugera Amps. Before they instituted and extended their very limited warranty to three years. They knew they purchased a Yugo when they bought the amp. Uli Behringer reputation isn't exactly a secret, he certainly deserved all the law suits. But some uninitiated fell into the amp through ignorance, attitude, or lack of performing due diligence and reaped what they sowed. "What's a Bugera in a landfill? A blessing!"

There's more fun in building a new amp than repairing the flaws of poor engineering.

Oh, I don't believe there is such a thing as too much information. Think of it as an introduction and stimulus to perform research. And sometimes, others write in with different opinions. This broadens horizons.

Best regards,

Steve


PS My Bugera anecdote, I received a telephone call to my cellphone on a Sunday from a stranger who had been given my name from an Electronics Surplus store I frequent when in Riverside, CA. He just purchased a new amp straight out of the box, not working. The store gave my number to him the previous day as he needed someone with a tube tester. He didn't bring over the amp to my house, just the tube, a Bugera 6L6 GC which the getter hadn't been flashed, the barium was still on the getter. I gave him a spare used Mesa 6l6GC STR which I new was good, and recommended he go to the store (it was a gift) where it was purchased to have them check out the amp.