JCM800 6550 tube substitution and fuses

[misterc57]

I picked up a JCM800 2204 with a pair of 6550 tubes. One of the tubes is bad. I have several matched EL34 tubes in stock so I am wondering if I can substitute with EL34. If I do so, do I need to do anything besides a bias adjustment?

Also I need to replace the fuses. Do I use fast or slo-blo?

They are 500 ma and 3 amp fuses. Are the radio shack .5 AMP and 3 AMP 250 Volt fuses ok to use? They only have slo-blo for the .5 AMP fuses.

Thank you! MC

[pdf64]

http://www.schematicheaven.com/marsh...l_50w_2204.pdf
Above schematic show both fuses as T (= time delay = slo blo).
But the line fuse is T2A, not 3? Does the back panel specify 3? I can't see any way 6550 should need a larger line fuse than EL34.
You will need to change both output tubes to EL34 (hopefully that's obvious, but best to note it).
I think that the conversion to 6550 involved changing the grid feed resistors from 220k to 100k, and replacing one of the fixed resistors in the bias supply, so as to increase the available volts.
So compare what you've got to the schematic, though the grid feed resistors are probably better left at a lower value.
Peter.

[Enzo]

.5A and 500ma are the same thing. Sorta like 6 inches and half a foot.

And as pdf said, they are SUPPOSED to be slow blow.

[misterc57]

>I think that the conversion to 6550 involved changing the grid feed resistors from 220k to 100k

Hard to read... Is that R24 and R25?

Thank you. MC

[misterc57]

3 AMP

Yes the back of the amp states 500 MA and 3 AMP for the fuses.

Is there a different schematic out there for 6550 tubes in a 2204?

Thanks again. MC

[pdf64]

http://www.schematicheaven.com/marsh...d_50w_100w.pdf
Above schematic (2nd page) shows what the EL34/6550 variations were, and is relevant to 2204 (see 1st page), though it's probably a different revision as the resistor numbers are different.
Whatever the resistor numbers are, it's the grid feed resistors to the output tubes which are 22k for EL34, 150k for 6550.
And the resistor that feeds the bias pot gets changed, 56k for EL34, 47k for 6550.
For the difference any of it would make, I would just put the EL34s in and see if they bias ok. If so then just leave it as it is, without changing any of the above resistors.
Peter.

[misterc57]

Mine is USA variant

My R24 and R25 is 150K

My R30 is 120K

My R22 is reading 4.47K in circuit.

Anyway.... I will try leaving everything as is (like the sound of that) and see if I can bias a pair of EL34.

Thanks! MC

[misterc57]

switch problem, bypass?

Please excuse all my basic questions but this is all new to me and I have a lot of interesting (to me) projects going on at once.

I got new fuses today, .5 AMP and 3 AMP. Rigged up my bias tester. Plugged in the amp, turned the power switch on, and waited and observed the el34 tubes light up. After a good 45 seconds I flicked the stand-by switch to the run position and nothing happened on my meter while monitoring the bias (stayed at zero).

I disconnected the power cord and tested the stand-by switch. I am not sure how these switches work but closest to me are two connectors parralel to each other with a blue wire in each. In the OFF position I get an open reading on my meter which makes sense. In the ON position I get 80.8 ohms resistance. I am guessing this should be under 1 ohm reading. But again not sure how this switch is suppose to work. Also not sure about the other contacts and wires hooked up to this switch.

My guess is that the stand-by switch is bad and I should be able to run a jumper wire across the two connectors with the blue wire to confirm. If so does that satisfy the switching function and the other connections to this switch or would I need to jumper other connections also?

Thank you again! MC

[misterc57]

where do I test for voltages?

I realized there is nothing wrong with the stand-by switch.

The tubes light up with the power button, but nothing happens when the standby switch is turned to the on position. My guess is that the power tubes are not getting high DC voltage.

I am trying to figure out the logical order on where to test for voltages and what type of readings I should be seeing.

I also have a variac available if that is helpful.

Thank you, MC

[Enzo]

First make sure your high voltage fuse did not blow already. The new fuses you got, ARE THEY SLOW BLOW TYPES?

[misterc57]

fuses are good

Fuses are good and they are both slo blow

[Enzo]

Set your meter to DC voltas, bround the black probe to the chassis, and measure at both terminals of the HT fuse holder. Got 500v at each end? Or 475 or whatever.

No? Go back to the standby switch then and measure AC volts between the two red (?) wires (from the transformer) and between each of them and ground. get roughly 700VAC between them and 350VAC to ground from each? More or less.

Got DC at the fuse? OK, then check for the same DC voltage on both pins 3 AND 4 of each power tube.

I dioubt it is missing but might as well also check for -45VDC or so on pins 5.

[misterc57]

VDC readings

+475 VDC on both sides of the HT fuse.

1st power tube (closest to fuse holder, not sure of the valve number)

pin 3: +469 VDC
pin 4: +.01 VDC
pin 5: 0 VDC

2nd power tube

pin 3: +.07 VDC
pin 4: +.014 VDC
pin 5: -.003 VDC

I am using the following schematic http://www.schematicheaven.com/marsh...l_50w_2204.pdf

Checked a bunch of local resistors tested in circuit, the ones varying from spec include

R26 (56K) reads 46.6K
R24 (228K) reads 157.1K
R25 (228K) reads 148.5K

R22 (100K) reads 4.47K
R30 (220K) reads 103.6K

The resistors attached to the tube socket read well

R31 (5K) reads 5.65K
R32 (5K) reads 5.72K
R33 (1K) reads .963K
R34 (1K) reads .996K

RV1 (22K) reads 21.6K

[Enzo]

Stop at the first tube. You have B+ on the plate but no screen voktage - so that tube won't conduct. the other tube seems to have neither plate nor screen voltages. That is suspicious. I'd expect the screen voltage missing, but the missing plate voltage either means your output transformer is open or your meter probe was not making contact. or you knocked your black probe off ground.

Look at your schematic. if you have B+ at the plates - pins 3 - but not at the screens - pins 4 - then the only thing between them is the choke. Check it for open or broken connection.

And if your -45v is missing from pins 5, then youneed to get that online as soon as possible. The tubes won't work happily without it. That could have caused the tube failures.

[misterc57]

Perplexed....

Maybe I did have a bad ground on the probes because this morning the readings have all changed.

1st power tube (closest to fuse holder, not sure of the valve number)

pin 3: +446 VDC
pin 4: +445 VDC
pin 5: -67.5 VDC

2nd power tube

pin 3: +447 VDC
pin 4: +445 VDC
pin 5: -67.7 VDC

I read 107 ohms on the choke.

I plugged an instrument cable into the input jack and was able to hear some noise through the speaker when grounding the tip of the cable with my fingers.

I plugged my bias tester back into the power tube socket and I am only able to read .006 mA when setting the trim pot to its max position. If I set the trim pot into the min position I get 0 mA reading.

I believe I am looking for 26 to 36 mA bias. So I am way off here.

Does this bring me back to the resistor changes mentioned previously?
If so quoting pdf64...

"Whatever the resistor numbers are, it's the grid feed resistors to the output tubes which are 22k for EL34, 150k for 6550.
And the resistor that feeds the bias pot gets changed, 56k for EL34, 47k for 6550."

The grid feed resistors, are these r24 and r25? I am reading about 150k on those.

Resistor that feeds the bias pot... is that r26? I am reading 46.6k.

Thank you! MC