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Tube amp, sudden loss in volume - need help


rhrocker

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I've never had a similar problem, and need help....

 

I bought a new Hughes & Kettner small tube amp combo recently featuring two EL84 power tubes and two preamp tubes. Suddenly, while playing at bedroom volume level, the volume dropped about 50%, and the amp now sounds weak and bad.

 

So I opened it up, and looked at the tubes while turning up the volume. All four of them look OK, orange (not red), like they should, right? Then I tried tapping each tube with my fingernail. Seems like one of the preamp-tubes "amplifies my tapping".

 

I need help, suggestions, some diagnosis. Should I try swapping the preamp tubes? I like this amp, and did not expect this to happen this early :(

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Yes, I forgot to mention that I am using the effects loop for delay and reverb.

 

Anyway, changing places for the preamp tubes seemed to do the trick. It works! I going to assemble it, and test it more properly.

 

Thanks for the suggestions! Maybe I should buy new preamp tubes (is this some kind of indication)?

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If just moving the tube around fixed it, it was probably loose in the socket or something.

 

I just had a similar prob with one of mine, lost alot of volume and the gain got fuzzy in a weird in-and-out kinda thing. Monkey'd around with it for an hour, and while pushing the meter probes all over everything, noticed that when I pushed on the pin3 prong on the bottom of the v2 preamp socket, it cleared up. When rocking the tube, all the other prongs would rock along with the tube, but pin3 was stationary.

 

Ends up that it had too much solder, and was stuck at a bad angle, and would occasionally lose most contact with the tube pin. Grabbed some desoldering braid and mopped up the solder to free up its motion, and boom, the amp sounds better than it ever has. There was always a weird grainy abrupt transition from overdrive to clean as notes sustained away, and that is FINALLY gone. Heh. Wish I'd have noticed that 2 years ago, it was so subtle, I always thought it was just a bad amp. Freakin' sings now, YEAH! It's cool that it finally went and broke most all the way, or I'd have never figured it out.

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Thanks for your reply BlackElk. Presuming you have the same amp, the 20W Edition Tube, it is possible that there is a problem with my socket as well... sometimes loosing contact. I'll check it out if it happens again.

 

For now it works like a charm. I read a tip about moving the tube around somewhere, that's why I did it.

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  • 13 years later...
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I have a soundcity 50h mark6, but the volume is too low, the volume set at level 3 of the master volume is no longer changing, all six tubes are good, the volume is low, and the tone is good. I can not study because I can not find the schematic. What I felt strange after many observations is the state of the output transformer.

 

Between the B + tap and each plate tap, 22.4 ohms and 26.2 ohms are displayed, and on the secondary side, between the COM tab and the other two tabs (the output transformer is completely disconnected from the amplifier and speaker) 0.3 ohms and 0.5 ohms were measured.

 

And, when AC6.3V is connected to the secondary side, it seems that the voltage is generated according to the respective winding ratio on the primary side.

 

But I think this is different from the knowledge I have known for a while.

 

It is not understood that the values ​​of the primary and secondary resistances are too low.

 

So, the point of my question is,

 

1. Is this output transformer normal?

 

2. Does this output transformer affect a large loss of volume?

 

3. What if the cause of the volume loss is something else?

 

4. Does anyone have circuit diagrams related to my amplifier?

 

It is a long story, but I hope for help.

 

Thanks!

 

I wish you all a healthy and happy time

 

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Hello rhrocker.

 

Glad to hear that things are back in order.

 

Just a kindly suggestion. When you post an issue that you are having (in this case your guitar amplifier) list the name and model of the equipment in question so that we can determine more quickly a solution.

 

You state in your initial post that your Hughes & Kettner combo guitar amplifier has 2 EL84 power tubes (considered by some to be a middy, crunchy and bright tube found in the VOX AC30) and 2 preamp tubes in it. A typical garden variety single ended EL84 makes a plate dissipation per tube of 11.5 watts at a plate current of 32.1 DC milliamps. Here the cathode bias is summed in series making a total of 23W.

 

Aside from all this minutia, you state that you are using the effects loop for delay and reverb, then follow by saying "Anyway . . .". :) My friend whichever device you insert into the effects loop that is interrupting your signal path by basic physics will find you at its mercy and could have been the problem, but thankfully not in this case. In troubleshooting this type of problem I would have suggested to employ the KISS method and strip everything down to only the guitar's output signal (no devices) to the guitar amplifier's input signal and eliminate the effects loop completely and always make sure to first check your mains fuse(s) too. Then proceed from there.

 

In the end the following statement makes no sense to me when focusing on the output of the guitar amplifier: "Anyway, changing places for the preamp tubes seemed to do the trick." Preamp tubes in a guitar amplifier have no key role in the power output of a guitar amplifier to its speaker driver(s). "Preamps" (meaning before the power amp stage) are used to drive the cable signal (or wireless signal) to the main output stage in a way such that the signal will not significantly become degraded known as the signal-to-noise ratio (SNR).

 

As for purchasing new preamp tubes this is more than likely not related to your problem here. Preamp tubes can typically last up to 20,000 hours – they don't get hot and function in a linear state, unlike class AB waveform power tubes. Class AB speaker guitar amplifiers offer high signal-to-noise ratio (SNR) and a low total harmonic distortion (THD + Noise). A friend of mine at Bob Pletka's Eurotubes has had 2 Mullard 12AX7 NOS preamp tubes burning now for over 32,000 hours and still emitting no microphonics. The usual and most simple procedure to check if a preamp tube is microphonic is to use the tip of a pencil to gently tap the end of each individual preamp tube and listen for a sustained ringing like a bell. If trouble is not apparent right away, turn up the volume and master volume knobs and keep tapping until you locate the tube(s) that rings like a bell or squeals.

 

Power tubes are completely different depending on how much you play (hours) how hot you set the cathode bias and the surrounding physical environment.

 

Still your final statement to your solution leaves me slightly baffled.

 

Rock on . . .

 

 

 

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I have the 50 plus which uses a dual power switch instead of a master volume. It apparently takes one gain stage out of series when the switch is flipped. I'd suspect a master volume might be located in the same general area.

 

If the master volume isn't working any more it may be the pot's going bad or more likely a tube gain stage has gone bad.

If you haven't changed the tubes in a long time, I'd at least change all the preamp tubes. If you change the power tubes you'll need to have them biased.

 

Here's most of the schematics. http://soundcitysite.com/schems.shtml I suspect the 50H will be similar enough to the other 50W amps where they didn't need to post a separate one for it. It does have more knobs then the 50 plus but tube amps are pretty simple designs so it cant be that hard to figure out.

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  • 10 months later...
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I  just picked up a Jet City 100 at an auction . it is dead mint . I powered it up ,had just started plucking on the clean channel and went to switch to the overdrive Me lost 99% of the volume on both channels. Wide open this thing just barely makes any sound. The distortion of the overdrive is there but it is so quiet can barely be heard. Tried cleaning fx loop plugs, swapped all tubes. Nothing. Any ideas?

 

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  • 1 month later...
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i have a peavey valveking 20MH tube amp head. ive had for 2 years now when suddenly the clean channel got distorted and muddy. it wasnt like that before. i tried to plug my guitar on return and send tone was crystal clear. do i need to change my tubes? it still has bright orange color. thanks

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46 minutes ago, zdk said:

i have a peavey valveking 20MH tube amp head. ive had for 2 years now when suddenly the clean channel got distorted and muddy. it wasnt like that before. i tried to plug my guitar on return and send tone was crystal clear. do i need to change my tubes? it still has bright orange color. thanks

The bright orange color just means the tube filament is still working... they can still have problems. If you plugged into the return jack and got a clean signal, then it sounds to me like one or more of the preamp tubes is bad. There are three 12AX7 tubes in that amp if I recall correctly. If you have a known good 12AX7, you can try using it to replace the three in the amp, one at a time, and testing it. You may only need to replace one of those tubes - if so, swapping in the good one will eventually tell you which one of the three in the amp are bad. Or you can just replace all three with new 12AX7's. The two power tubes (EL84's) sound like they're fine - you wouldn't get a nice clean sound by plugging into the effects return if they were the source of the problem. 

 

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