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Author Topic: JMP 2100 not making power  (Read 7317 times)

ilyaa

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JMP 2100 not making power
« on: March 26, 2014, 10:52:05 PM »
so im trying to put some of my new skills to use -

someone gave me this amp to look at  - "sounds quiet"

B+/plate voltage - ~385VDC

tubes are EL34s
i did a quick bias, trying to get 14 watts or so dissipation.
started with one tube - looking for about 37mA. got it there with about ~-31VDC on the grids.
checked the other tube - it was pulling about 50mA.
switched them around to see if tubes or sockets and the readings moved with the tubes.
time for new tubes?

i then checked the power output with a 100mV sine wave into my (brand new) dummy load - it was giving me 12V p-p - so 4V RMS, so it's only making about 4 watts (into 4 ohms)?!?!?

this is a 50 watt amp...

only other thing that looked suspicious is the ripple on the main filter cap - about 8V p-p. is that acceptable? seems high....i thought the plate voltage seemed a bit low....

how am i doing?

(as far as i can tell the attached schematic corresponds to this amp...)

« Last Edit: March 26, 2014, 10:53:32 PM by ilyaa »

Roly

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Re: JMP 2100 not making power
« Reply #1 on: March 27, 2014, 03:00:17 AM »
Quote from: ilyaa
i then checked the power output with a 100mV sine wave into my (brand new) dummy load - it was giving me 12V p-p - so 4V RMS, so it's only making about 4 watts (into 4 ohms)?!?!?

The input signal level doesn't mean much; the important thing is that you measure the output power at the onset of clipping.  In this case it is likely to occur a lot earlier on one side than the other, and combined with overall low power output, and the lack of any other signs of trouble (i.e. the signal voltages coming out of the Phase Inverter are about the same as the bias voltage, so there is no lack of drive) then it is reasonable to suspect that the output valves have worn out and need to be replaced.  This is very common, and normally all you have to do.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: JMP 2100 not making power
« Reply #2 on: March 27, 2014, 10:56:24 AM »
Quote
at the onset of clipping

yeah i turned it up until the sine wave started to wonk out a little bit.

the PI: you mean the AC voltages coming off the plates to feed the power tubes? these should be close to the DC bias voltages on the power tube grids?

g1

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Re: JMP 2100 not making power
« Reply #3 on: March 27, 2014, 12:05:45 PM »
  Any chance you have an x10 scope probe?
The PI plate signals (peak to peak on scope) should be about double the bias voltage to get full power.

ilyaa

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Re: JMP 2100 not making power
« Reply #4 on: March 30, 2014, 05:18:21 AM »
k so i got new el34s, popped them in, but everything is *not* better.

1) even with the bias voltage as high as it will go (about ~30VDC), the tubes are only pulling 30 mA or so idle current. if i swap the old tubes back in, they pull way more. ??. the amp chassis says 6550 on it, and ive read that these amps were kind of made for those tubes. is there something i need to change in the bias circuit for el 34s to work? if that's the case, though, can someone explain why the old tubes (also el34s) could pull so much more current? but still make low power for the amp....?!

2) measuring at the plates of the PI, i only get about 40V p-p. that is definitely less than twice the bias voltage. ??

3) ive got about 400 VDC on the power tube plates and pin 4s. right heater voltage, too. no other suspicious stuff that i can see.....


DrGonz78

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Re: JMP 2100 not making power
« Reply #5 on: March 30, 2014, 06:14:48 AM »
I have only read about such issues with these types of amps so take my advice with a grain of salt. However, there are versions of these amps where there were 6550 or KT88's for the output tubes. The bias resistors for EL34's would be the 220k ones that you see in your schematic. However, if you had 6550/KT88 type then those resistors would be 150k. So it would be wise to look at your amp and locate those resistors to see exactly what we are dealing with here. If those are 150k resistors then yeah the tubes won't bias correctly with EL34's. Good luck. Look at the schematic attached...
“A person who never made a mistake never tried anything new.” -Albert Einstein

DrGonz78

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Re: JMP 2100 not making power
« Reply #6 on: March 30, 2014, 06:21:15 AM »
Please note that I am wrong calling those 220k resistors "bias resistors" and I believe actually they are called grid leak resistors. However, the bias circuit could be wrong in general terms and you need to start by measuring/confirming all the values in relation between both schematics on this thread to your amp.

Edit: The grid leak resistors are not the main problem to bias the outputs as the one single 150k or single 220k resistor that is in the bias circuit. Not sure also if the Bias trim pot is different but I don't think it is...?
« Last Edit: March 30, 2014, 06:50:51 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: JMP 2100 not making power
« Reply #7 on: March 30, 2014, 07:38:57 AM »
Quote from: ilyaa
2) measuring at the plates of the PI, i only get about 40V p-p. that is definitely less than twice the bias voltage. ??

Ah, you should have said that sooner.  That doesn't look right to me, a differential amp PI should be capable of a lot more signal swing.

A quick LTSpice suggests that it should just be starting to limit with 4 to 5Vp-p in, at which point you should be getting around 180Vpeak-peak on each anode.

Now the question is, is this the PI's fault, or is it getting insufficient drive from back up the signal chain?

Check that you have ample HT on the feed point for that stage, and the next obvious thing to try would be a known good ECC83/12AX7, however measure the voltages around the PI and see if they look sensible - if they do then the problem is likely to be earlier in the signal chain.

With the EQ all set for 50% you should be able to inject a signal into the wiper of the treble control and be able to drive the OP stage to full wick (it will take a few volts, and the OP stage should clip long before the PI).

Check that the anode voltages in each preamp triode section are reasonable (i.e. a bit over half the local HT).

If you still aren't getting any joy I'd try injecting a signal in the front end, say around 100mV, and check that you are getting something like x30 voltage gain from each triode section (except the cathode follower feeding the tonestack of course, that will be close to unity).
If you say theory and practice don't agree you haven't applied enough theory.

g1

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Re: JMP 2100 not making power
« Reply #8 on: March 30, 2014, 01:54:28 PM »
  The bias range will be greatly affected by the resistor Dr.Gonz mentioned, at the cathode end of the bias diode.  It and the 2 resistors at the power tube grids should be changed for the type of output tubes:  150K in all 3 positions for KT88, 220K for EL34.
  As far as the low volume, a good place to start is DC voltages for pins 1,3,6,&8 of V1,V2, and V3.

ilyaa

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Re: JMP 2100 not making power
« Reply #9 on: March 30, 2014, 02:20:45 PM »
yeah i think its set up for 6550, according to the bias network.
but still, can anyone explain why two different sets of el34s would be allowing such drastically different cathode currents?

DC voltages:

V1:
pin 1: 100V
pin 3: 0.7 V
pin 6: 140V
pin 8: 1.4 V

V2:
pin 1: 120V
pin 3: 0 V (!)
pin 6: 220V
pin 8: 129 V(!!)

V3:

pin 1: 200V
pin 3: 34 V
pin 6: 200V
pin 8: 34V

injecting a signal straight into the treble control i can still only get about 60V p-p off the PI, though....doesnt this bypass V1 and V2, anyway?
« Last Edit: March 30, 2014, 02:30:23 PM by ilyaa »

g1

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Re: JMP 2100 not making power
« Reply #10 on: March 30, 2014, 08:55:28 PM »
  The difference in bias of the power tubes could just be variation in the tubes, or the old ones could be faulty.  Once you change the bias circuit to the proper resistors for EL34 you can get a better idea of how much difference there is (how much difference in grid voltage to get the same cathode current).
  Yes, injecting your signal at the treble pot wiper bypasses V1&2.
So what is happening when you hit 60Vp-p?  The waveform is clipping?  Do you still have lots of room to increase the output of the generator?
Try it with the power tubes removed and see if you get more than 60p-p.

P.S. You said you were only getting 40Vp-p, but now you are getting 60Vp-p, which is double the bias voltage if it is at -30V.  Is this the case?
And when the PI is putting out 60Vp-p, what voltage do you get at the load?
Also, V2 pin3 appears to be a measurement error, recheck.
« Last Edit: March 30, 2014, 09:04:31 PM by g1 »

ilyaa

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Re: JMP 2100 not making power
« Reply #11 on: March 31, 2014, 05:11:38 PM »
okay:

putting in a 1khz 100 mV sine wave into the input (with no power tubes installed) -

stage one (measured at the anode): 2 V p-p
stage two (measured at cathode): 45 V p-p (with volume cranked)
stage three (measured at anodes): 100 V p-p (starts to clip)

should stage two be at unity? overall this seems like okay gain, right? about x1000. thats a big swing off the PI, right?

putting a 1khz 1 V sine wave into the treble control (no power tubes) -

stage three (measured at anode) 100 V p-p (starts to clip) - about the same.

putting in a 1khz 100 mV sine wave into the input (with power tubes installed) -

stage one (measured at the anode): 1 V p-p
stage two (measured at cathode): 30 V p-p (with volume cranked)
stage three (measured at anodes): 75 V p-p (starts to clip)
voltage at the 4 ohm load: 35 V p-p


this is with the not properly biased el34s in. i have some 6550s i will put in later on (a friend is bringing them). seems like the amp might be in okay shape, overall though, right? maybe just needed the preamp tubes moved around (which i did).

also i rechecked and yes, i get about 0.7V at V2 pin 3. but all the other DC measurements are the same as i posted previously.

strange detail: the presence knob has been re-wired on this amp. instead of being attached to the grid of the PI, it is wired as a rheostat between the two 22nF caps on the PI anodes. if this knob is all the way down, i cant get nearly as much unclipped power out of the PI as I can if i crank it up. what is this modification?

Roly

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Re: JMP 2100 not making power
« Reply #12 on: April 01, 2014, 09:59:08 AM »
Quote from: ilyaa
it is wired as a rheostat between the two 22nF caps on the PI anodes. if this knob is all the way down, i cant get nearly as much unclipped power out of the PI as I can if i crank it up.

If I understand you correctly, a variable resistance simply wired between the anodes of the PI (no cap in series) will only shunt/short the drive voltage to the OP stage, so what you observe is what I'd expect.

Quote from: ilyaa
what is this modification?

um ... clueless?  It seems like a mighty curious mod to me.

If you look at the circuit the NFB is applied from the OPT secondary via the 47k(100k) resistor to the cathode circuit of the PI, and the cap on the pot wiper acts as a top cut on the NFB.  If you top cut NFB you end up with top boost, but if you don't have a cap at all you don't have a frequency selective component and the knob loses any claim to being a tone control.  Unless you have a compelling reason to leave it I'd restore it as original because this will certainly limit available OP power by limiting the drive into the OP grids.

Quote from: ilyaa
voltage at the 4 ohm load: 35 V p-p

P=E2/R

35pk-pk = 17.5Vpk = 17.5/1.414 = 12.4Vrms

(12.42)/4 = 38.44 watts, which is not unreasonable, but I'd expect more like 50-60 watts from a pair of EL34's in fixed bias.

Are you sure this is supposed to have a 4 ohm load and not 8 ohms and/or it is on the right OPT tapping?
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: JMP 2100 not making power
« Reply #13 on: April 01, 2014, 11:33:59 AM »
yes - the mod on the PI output has a direct impact on the output voltage. i think i will restore it - is it some kind of strange effort to put a master volume on the final stage?

it has an output selector. it was set to 8 ohms originally but i only made a 4 ohm dummy load (what a dummy) and so ive switched it to be on 4 ohms. should i trust the selector? if not, can i measure resistance on output transformer leads to verify?

Quote
(except the cathode follower feeding the tonestack of course, that will be close to unity).

should that stage have been giving me unity gain?
« Last Edit: April 01, 2014, 11:55:14 AM by ilyaa »

g1

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Re: JMP 2100 not making power
« Reply #14 on: April 01, 2014, 05:14:53 PM »
  The cathode follower is not a gain stage, it is for driving lower impedances like the tone stack so it is fine.
Which side of the 22nF caps was the pot on?  At the PI plates or the power tube grids?
  It's acting as a master, but will only go to zero if the PI is perfectly balanced.  As that is not likely, you will still get some sound with it turned down all the way.
What did they use to replace the presence pot, just a 5K resistor with no cap?  That would be like having the presence turned all the way down all the time.
  Check with the owner if he uses that "master", if he does, I would recommend moving it somewhere and putting the presence circuit back to stock.