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Problems with London City Bulldog / Ashton VP100 tube amp
Problems with London City Bulldog / Ashton VP100 tube amp
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Old 25th March 2020, 08:36 AM   #1
Ricotjuh is offline Ricotjuh
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Default Problems with London City Bulldog / Ashton VP100 tube amp

In short, I have a London City Bulldog tube amplifier. However, this is a "cheap" copy of a Marshall JCM2000 DSL100, which have been marketed in different versions. Another well-known is the Ashton VP100. A slightly different jacket, but identical on the inside to the London City Bulldog.

Now years ago I had the problem that the amplifier pumped loud pops and rumble from speakers when the standby button was turned on.
Turning the level knob all the way to "0" did not change the volume at all. The creaking and popping remained just as loud.
Can't fix the problem at the time. Then put the amplifier in the corner and no longer used. Now I want to get the amp to work again and I have dusted it again. Now the "noises" seem to be as good as gone, but I do hear a constant hum coming from the speakers. This cannot be eliminated by turning the level to "0".
The volume has dropped drastically. Now I have to turn the level knob more than half way to get a little sound. Where normally the whole street would complain if it was on this level.
Then disassembled the amplifier and now I see next, a number of suspicious capacitors. 2 of which have already burst from the jacket.
Click the image to open in full size.
The capacitors that have clearly visible burst are the capacitors for the heaters of V1 and V2.
With the other blue capacitors I see no bulging over mess around the capacitors.
To be on the safe side I replaced all the capacitors.
Unfortunately, there is no improvement yet regarding the constant hum coming out of the speaker and the huge dip in the volume.
In any case, I have now excluded that it is in the capacitors. I also measured on the points indicated in the diagram. See image below. instead of +444V I measure +464.6V and where +432V I measure +462.3V. Where I should measure -36V, I measure -39V.

+464.6V compared to +444V seems to me within the margins.
However, I find the difference of +462.3V and +432V. Because the difference in the schematic between those 2 points is 12V. And I only measure a difference of 2.3V.
Is this curious?

What is even more remarkable (or not) is that there is a loop level switch on the back. And when I put a jumper cable in the send and return, I don't notice any difference when I switch this switch on or off.
When I put a pedal between the send and return, it boosts the sound. Is this correct?

I know, there are a lot of questions but I don't know where to look ...
So briefly the problems I have:
1. Volume dropped drastically
2. Constant hum on the speaker. (does not respond to knobs)

Does anyone have experience with this amplifier or related or can someone help me with troubleshooting, I would like to hear it.
Thanks in advance.
Click the image to open in full size.
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Old 25th March 2020, 09:07 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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It is only a basic valve amplifier.
The noises are probably an EL34 failing.
The effects loop does as you describe, the volume varies with the type of pedal used.
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Old 25th March 2020, 09:27 AM   #3
Ricotjuh is offline Ricotjuh
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I biased the tubes at 70mV across R107 and R108.
How can I best test if one of the EL34 fails?
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Old 25th March 2020, 12:26 PM   #4
Ricotjuh is offline Ricotjuh
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Just measured the current through all the cathodes. V5=34mA, V5=37mA, V6=37mA and V8=37mA. I think this looks good.
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Old 26th March 2020, 07:30 AM   #5
Diabolical Artificer is offline Diabolical Artificer  United Kingdom
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Measure the HT with your meter on AC volts. Have you got a sig gen and scope? Soundcard scope is a good free PC based sig gen, scope etc if not. Trace a signal from IP to OP, big loss of OP sounds more like a voltage amplification stage, but it could be anything from a dry joint to out of spec resistor to a tired valve, hard to tell from this distance.

Andy.
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Old 26th March 2020, 07:46 AM   #6
Ricotjuh is offline Ricotjuh
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Quote:
Originally Posted by Diabolical Artificer View Post
Measure the HT with your meter on AC volts. Have you got a sig gen and scope? Soundcard scope is a good free PC based sig gen, scope etc if not. Trace a signal from IP to OP, big loss of OP sounds more like a voltage amplification stage, but it could be anything from a dry joint to out of spec resistor to a tired valve, hard to tell from this distance.

Andy.
Yes I have a signal generator and oscilloscope. Okay, so I put a 1kHz signal on the input and measure from the input to the output? Of course I have to take into account that when I measure the preamp tubes on the anode, I do this after the decoupling capacitor. Isn't this right? Otherwise my scope is not going to like it that much, I expect.
For V1a this is after C6.
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Old 26th March 2020, 10:01 AM   #7
Ketje is offline Ketje  Belgium
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Did you try another 12AX7 for V4 allready ? (exchange with one of the others)
Could be a leak heater-cathode.
Mona
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Old 26th March 2020, 10:16 AM   #8
Ricotjuh is offline Ricotjuh
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Quote:
Originally Posted by Ketje View Post
Did you try another 12AX7 for V4 allready ? (exchange with one of the others)
Could be a leak heater-cathode.
Mona
Not yet. I can try that too.
I can see that for V3 and V4 a 12AT7 has been used instead of a 12AX7.
While the schematic only prescribes a 12AT7 for V3. Is there a good reason to use a 12AT7?
I still have new 12AX7 tubes. Can it just be changed for that?
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Old 26th March 2020, 02:51 PM   #9
Ricotjuh is offline Ricotjuh
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Incidentally, just swapped V3 with V4 (12AT7). This made no difference.
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Old 26th March 2020, 07:17 PM   #10
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Ricotjuh View Post
Incidentally, just swapped V3 with V4 (12AT7). This made no difference.
You can't just put an 12AT7 in for V4 without changing R93 (470Ω).
That tube needs ~1k2 for R93.
I guess the common cathodes are ~45V above ground if all is well.
Mona
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