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

[donzoid]

Thanks Fire, yes I got that part...but if I wanted to change the range to accommodate differing tube types, then couldn't I change that value (at that location) ...or would I have to do that after the caps (where the piggyback is now). I think I'd better get myself a "theory for dummies" book of some sort! I realize this is still AC before the diodes, but thought perhaps that might work as long as the calculated sum was right for the tube type. Even if not, it would still clean up the PCB a lot closer to stock.

I certainly get what Ren is saying about the conversion from the existing PCB to a turret...lots of people do that, but if I'm gonna do that...I'd rather start by doing it on my 50 watt club workhorse! Plus buying a resistor is reasonably cheap vs. even the MC transformer for the short-term fix to get it in place again as my backup for a few more paying gigs...then move to a new PT ... even if its a cheaper one THIS TIME I will do the work and make sure it meets intended spec!

[martin manning]

There is probably not a thing wrong with the Partridge PT, and since it has the extra tap it would be easy to clean up and adjust the bias circuit as I described above. The range can easily be shifted as needed for tube type by adjusting the resistance between the PT and the diode. If I were you I think I'd do that and play on.

[donzoid]

I picked up the resistors today...will try this shortly (tomorrow by now). All they had in stock were 82k and 91k. I got both in case I have to swing a bit either way to get close to center on the trim. Of course I will report on results as soon as I try it!

[martin manning]

Hold on... If this 250V tap is on the primary side of the PT (so says Donzoid in a PM) it would work but the bias circuit would not be isolated from the mains, and so it would NOT be a good idea to use that. Sorry to get your hopes up if it is, and it's a good thing you asked. Just to be sure, though, see if you have continuity from this 250V tap to the 120V primary connections.

Deriving bias voltage from the HT winding on the secondary is a possibility, but since this amp has a full-wave-bridge rectifier (FWB) you need to use a voltage divider with a capacitor in the series leg to get an AC voltage that goes below ground. This method was used on some Marshalls, as shown in the circuit snip below. This means two additional parts (shown as C15 and R7) and a connection to ground are needed. You would adjust R7 to get the voltage you need. The HT and ground are close by, and maybe with some thought we can come up with a neat-and-tidy way to fit them in there. There are some holes and extra real estate on the board that could be used to mount these parts.

If this was my amp, I would take the bias from both sides of the B+

Steve, what's your thought here Re "both sides of the B+?"

[donzoid]

Well...I just "had" to ask didn't I?

My concern was that the wire may bypass the fusing by doing it that way and possibly smoke me or the amp. Glad I asked also. I have taken 440 before, and am well aware of "respect to ground". Fortunately it was only for a couple of seconds and all I got was a wakeup and big, burned hole in two fingers...

I'll check continuity this morning. It is possible that the tap comes off the secondary but I wanted to make sure.

Now, the 1969 Philips datasheet for el34 (Siemens ought to be somewhere close to this or Mullard) is -32 volts in class AB mode so even though we are on the edge of that with the tap being used now, is the real concern that this circuit gets "starved" when I put it to load (by playing it turned up, usually about halfway on the master)? Or do those values not change? I'm not sure that just adjusting resistors in the existing setup to get in the right range would not at least get this turkey workin' for this weekend.

[donzoid]

Oh...bother. Yes there is about 8 Ohms continuity between this tap and the mains.

[Firestorm]

Not to muddy the waters, but for under $20 you can get a 115v to 2.5v filament transformer. Just hook it backwards to the filament supply, bridge rectify with the positive output to ground and negative to the existing bias cap (disconnect and insulate the existing bias tap). That should give you around - 65v raw bias. You need very little real estate, just somewhere to mount one small tranny and the bridge. Filament supply probably has current to spare and bias doesn't take much.

[JazzGuitarGimp]

Now, the 1969 Philips datasheet for el34 (Siemens ought to be somewhere close to this or Mullard) is -32 volts in class AB mode so even though we are on the edge of that with the tap being used now, is the real concern that this circuit gets "starved" when I put it to load (by playing it turned up, usually about halfway on the master)? Or do those values not change? I'm not sure that just adjusting resistors in the existing setup to get in the right range would not at least get this turkey workin' for this weekend.

I don't see that a schematic for the amp was ever posted in this thread. So, not being familiar with the amp, I will say this in answer to your question: If the bias circuit uses the standard 220K resistor to each grid, then the maximum current required of the bias circuit is about 0.5mA, and that's assuming a worst-case bias voltage of -50V.

[donzoid]

I don't think its muddying the waters Fire...just another option to explore. And a good one really. At this point though, if I am gonna go spend money on a transformer, diode bridge and the time to drill, mount and wire it...good for the learning I admit...but I know there are all kinds of people watching this thread with interest. What's my least expensive (but at least decent) price on an Edcor or MC M-6 replacement? I'll bet someone reading this thread has one, or buys them in bulk for their amp builds--PM me! If no one has one they want to let go of, by golly I'll order one from MC in the next couple of days. Heck these Siemens EL34's are being compared to old stock mullard...maybe with the correct PT in there, I might change my mind about this type of tube.

I believe this is the correct schematic for the 2203 JMP but with all the mods done it's not gonna be exact...

[renshen1957]

If this was my amp, I would take the bias from both sides of the B+

Steve, what's your thought here Re "both sides of the B+?"

Hi Martin,

The "both side of the B+" is based on my preference of a full wave negative bias supply and a philosophy that merely adequate (half wave either from B+ or from a single bias tap) doesn't cut when the amp you built is being used professionally.

None are my amps to my knowledge are used as such, but I believe one should build as though each amp would be used every night for long hours and being stressed or abused.

A resistor network (voltage divider) and a duplicate resistor network is used to provide Bias from the HT. If either diode should short out, there is still adequate bias to the power tubes. Diode failures are rare, but these do occur. In a half wave bias set up (single diode), the fertilizer hits the ventilation system if a diode failure occurs.

I provide NOS power tubes or Hi Fi quality tubes with my amps. I recently read that Winged =C= factory closed, so it's either the usual suspects at New Sensor or to Psvane or Full Tone Music for quality new production. A Quad of Power Tubes for the Golden Ears market maybe expensive nowhere near what NOS Tele or Mullard EL34 cost. The early word on the Psvanes copy of the Phillips metal base by this crowd is these tubes equal the performance of the old Mullards. The tube base is made from Teflon with a metal casing.

Hi Fi drives the tube market where quality is concerned, and as a market the Audiophile purchasers have both the money and a very critical nature as to tube quality. Ruby and Mesa sometimes get good marks, but Groove Tube QC wasn't so hot for a long time before Fender purchased the company.

Because of the expense for NOS or Audiophile Tubes, an extra protection factor is an insurance and a better night's sleep.

Heat is an acceleration factor in diodes, and tube amps generate a lot of heat as we all know. Mains voltage spikes, (I use a Zener diodes as protection for the bias supply with a cap to eliminate the zeners' noise) can also accelerate diode failures; bars and other venues aren't always wired correctly, and only professional groups bother with power supply conditioning.

Those with an engineering background (I am not an engineer) will find the following interesting especially the math http://www.diodes.com/quality/reliability_handbook.html

There is also defects in manufacture to consider.

Theories on parts failure are all well in good. Real world observation is what concerns me. I am involved in a niche market producing power supplies as a subcontractor. I have purchased tens of thousands of full wave rectifiers from American companies (maybe made in the US, as claimed) and rectifiers that have been sourced from Asia. I have also repaired my predecessors products (same parts) under contract which date back almost ten years.

In a non-heat environment full wave rectified at low voltage with over rated parts, failures have been low from use. Defective components do occur in so many 10,000. But which one in ? The 10th, 230th, 5769th, or 9999th. With my luck, the critical component breaks when it is most needed, or when I send a sample to the boss.

Most component failures occur between the first 30 days or sometimes within one year of constant use. As those should be considered as 30 days times 24 hours.

How common are bias failures in tube amps? I just don't want to take the chance.

Yes, I know,,,,,,

I ramble on. The person who is the greatest influence on my building philosophy also was one in his amps to use full wave rectification. He was also one for individual bias for each Power tube.

Best regards,

Steve

PS I use Solen Fast Caps in Place of Electrolytic Caps in the Power Supply when space allows. The Solens should last the life of the amp and not need ever to be replaced.

[renshen1957]

What's my least expensive (but at least decent) price on an Edcor or MC M-6 replacement?

I believe this is the correct schematic for the 2203 JMP but with all the mods done it's not gonna be exact...

Hi Don,

Check out EDCor P=Transformer at http://www.edcorusa.com/p/589/xpwr002_120

XPWR002-120

Power transformer for a 120V, 60Hz. line to 720V (360-0-360) at 220mA center tapped, 60V at 800mA, 6.3V at 7A and 5V at 4A.

$57.33

Product is built to order.

Primary (input) Voltage 120Vrms, 60Hz.
Secondary (output) Voltage 720V(360-0-360)@220mA, 60V@800mA, 6.3V@7A & 5V@4A
Bobbin Material Glass Filled Nylon 6/6
Flamability Rating Class B 130°C
Core Material M6 29 gauge. line grain oriented steel
Termination UL1015, 18 ga. lead wire
Mounting Powder coated steel end bells
Weight 7.33 lbs.

Only one voltage option.

Two voltage options

http://www.edcorusa.com/p/794/xpwr173_120-240

Design
Mathematically designed to produce the specified voltage(s) when loaded.

Primary (Input)
Designed for 120V or 240V, 50/60Hz.

Secondary (Output)
This transformer has multiple voltage windings, 700V (350-0-350) at 400mA center tapped, 100V at 10mA, 12V at 1.5A and 6.3V (3.15-0-3.15) at 7A.

Magnet Wire
All units are wound with the finest virgin copper magnet wire. All windings use heavy insulated wire to provide excellent manufacturing yield, lower customer cost and superior life.

The Core
EDCOR uses the finest M6 line grain oriented steel cores. Each unit is hand stacked to assure at least 92% stack.

The Bobbins
EDCOR's royal blue bobbin with black finishing tape.

Termination
The terminations are UL1015, 18 ga. lead wire.

Enough heater juice for ten 12ax7 (at 12V) plus 7 A for power tubes.

$85.03


CLASSICTONE # TYPE REFERENCE #
40-18053 Power, 100W, 120/220/240V T2562 (Upright Version),T4145 (Upright Style Mounting)


Amps Used In: Most 100W JMP** and JCM800** type amplifiers (upgrade)

Description: Great vintage era type constructed, paper layer wound upgrade / general replacement power transformer! This is an upright mounted transformer. The multi-national primary voltages of 120/220/240 are featured here. This also features the upgrade of a flux band and M-6 steel lamination for the ultimate in cool running and high performance!

http://www.classictone.net/40-18053.pdf
1 piece @ $92.92 each


Compares to http://www.classictone.net/40-18068.pdf

CLASSICTONE # TYPE REFERENCE #
40-18068 Power, 100W, 100/120/220/230/240V 1202-43
(Laydown Style Mounting)


Amps Used In: 100W JTM45/100** Marshall* style amplifiers

Description: Great vintage era type constructed, paper layer wound power transformer for the 1966 version of the classic 100W JTM45/100**. This features the original, higher B+ rating option of 560V for the purist that wants authenticity but also features a lower, more conservative B+ option of 490V that is better for use with modern KT66s.

1 piece @ $151.01 each

http://www.classictone.net/40-18053.html



General Features:
TM
ClassicTone Part Number 40-18053

Best regards,

Steve

[donzoid]

Thank you Steve!

[martin manning]

Steve, thanks for the clarification on your bias supply comment. Re Psvane... $100 per tube? Nope. Interesting story on their site about their split with Shuguang, though. Capitalism is alive and well in China.

[donzoid]

Yeah...I went out last night and read that too. Having that kind of home audio equipment is God's way of telling you...you have too much money...LOL...but it SURE sounds good!

Oh and that factory they acquired, I think that may be where some of the old military Chinese tubes were made. If that's the case they may be onto some pretty high qual. production. Not sure without research if the old Beijing factory (where they made the military 12ax7's) is even still there. I have a dozen of those....they really aren't bad at all.

[renshen1957]

Steve, thanks for the clarification on your bias supply comment. Re Psvane... $100 per tube? Nope. Interesting story on their site about their split with Shuguang, though. Capitalism is alive and well in China.

Hi Martin,

Psvanne and TJ Full Music are building great tubes with highly skilled labor.
And the company is a private venture, so yes capitalism is alive in China. I wish the same could be said in the US.

I don't see New Sensor going to the same lengths that either Psvanne/TJ Full Music (box plate ECC83 CV4004 reproduction) has gone to produce quality tubes. New Sensors 7868 re-issue which has the wrong pin diameters and the 7591 has the wrong height tube compared to the original tube. At least New Sensor isn't producing the 7591XYZ (examine your zipper) anymore, a re-pinned 6L6 tube that wasn't electrically capable for the abuse 7591

Would you pay $13.85 for one of these tubes? Yes, but it would be in 1966.

That would be better than a day and a half's labor at minimum wage ($1.10 per hour). The US minimum wage is (7.25 in most states) since 2009 which equals 1.01 in 1966 dollars if not in buying power. A GE 6CA7 tube would cost $5.35 in 1966, $39.00 in 2013 money.

However, back then sales competition was fierce, tube demands were higher, and production was full blown to capacity for the military, TV, and other consumer product commands.

The Psvanne EL34 Metal base reproduction is expensive but it is still less than NOS Mullards, Real Siemens, (not RFTs), Phillips, or Telefunken Tubes. The NOS NIB Box (I don't trust "tests NOS") on Ebay are $500 per tube for Metal base, for the regular NOS NIB EL34 $185 per tube with a no return policy.

The better NOS Tube sellers charge about the same, with better policies, but to paraphrase Roy Rodgers, Mullard isn't making any more EL34 X2 tubes. Nor is anyone else. A labor intensive product that lost demand after Transistors with their economies triumphed, and the military stopped purchasing in bulk doesn't survive long under Capitalism

Psvanne does have a "Hi Fi" grade that's $136.00 per quad. Either the Hi Fi or the Century note tube does have a 90 day warranty for replacement.

Best regards,

Steve

PS a GE 12ax7 in 1965 cost $2.40

Price lists for 1966 GE, 1968 DUMONT, and 1986 Phillips ECG North America are at http://makearadio.com/tubelists