Tube Amp Tremelo

[h20rider]

An old HARMONY tube amp with tremelo. It's been all recapped. Bit "hummy" but works well except for the tremelo. It worked until the fuse blew. There's a jack for the footswitch, but the effect worked without any footswitch. Since the fuse blew and has been replaced, no trem effect available. I've inspected for loose connections oe signs of a short...nothing evident. Suggestions? Perhaps a tube? Since I haven't the slightest idea of how the effect is produced, I have exhausted my range of solutions. Again....any suggestions?

Thanks in advance.

[lovekraft]

Sounds like a bad tube - its untimely demise may have been what blew the fuse. If you can be a little more specific about what model Harmony it is, or at least what tubes it uses, we might be able to point out a likely suspect.

[Paul Marossy]

I haven't the slightest idea of how the effect is produced

There's only two ways that I know of:

1. Optoisolator (the "roach") used in Fender amps. Basically, it uses a light dependent resistor (LDR) to create the vibrato effect.

2. Power tube bias modulation. Bias modulation will make the tubes pulsate - they brighten and dim as well as the volume level rising and falling. I think this would be harder on the power tubes, though.

Anyhow, the first thing to suspect is the power tubes. Then go from there.

EDIT: BTW, if your star ground that amp, I bet it will be amazingly quieter. I did that to my Gregory Mark X (which sounds a lot like your amp circuit, topology-wise), and I was amazed at how much the hum was reduced - down to practically nothing!

Edited October 3, 2005 by Paul Marossy

[1guitarslinger]

Great advice.

I used to have an old Harmony amp a long time ago, till I did something really stupid (I was very young) and blew it up.

Visual inspection first is always the way to go. Look at the tubes when the amp is powered up. See if one is dark, or glowing too bright.

Swap the tubes with known good ones and see what happens. Probably time for new tubes anyway.

With the amp unplugged and the filter caps discharged, check to see if any of the components look burnt. "Poke" at everything with a chop stick or wooden skewer or something to see if any resistors or other components, or their leads are cracked.

Like LK said, it is probably a tube, or maybe the opto isolator as PM pointed out, or it could be something else...

Good luck with it.

EDIT AND DISCLIAMER: To all, don't go messing around inside a guitar amp, plugged in or unplugged, unless you are confident that you know what you are doing. It's worth it to pay a tech a few $ rather than get yourself blown up. Let the tech get blown up...that's what he's paid for! ;-)

Seriously, you can get hurt and/or damage the amp by the higher voltages. Be careful.

[lovekraft]

Paul, you think it's the power tube(s)? My first instinct was to suspect the triode that's the trem oscillator, especially since the trem failure was accompanied by the fuse blowing. Since neither of us has actually seen the amp, I realize that we're both guessing, but I was just curious about why you came to that conclusion. You know me, obsessed with the process, and not afraid to ask the really stupid questions!

Oh, and the slinger is correct, 'rider - standard disclaimers about potentially lethal voltages even in unpowered tube amps remain in effect, so be careful and don't get bit!

[1guitarslinger]

oops...Edit ;-)

[Paul Marossy]

Paul, you think it's the power tube(s)? My first instinct was to suspect the triode that's the trem oscillator, especially since the trem failure was accompanied by the fuse blowing. Since neither of us has actually seen the amp, I realize that we're both guessing, but I was just curious about why you came to that conclusion.

Well, I didn't look at a schematic for the amp, but I would think that amps which modulate the power tube bias to achieve a tremolo effect work the power tubes harder than something like a Twin Reverb, which is wroking a preamp tube. It could be either one not knowing what the schematic looks like...

[lovekraft]

Yeah, I wasn't questioning your judgement or anything, just wondering about what lead you to that conclusion, and if I was missing something. Someday we'll have to put everybody's notes together and write the quintessential troubleshooting manual (maybe we could get RG to edit - now that would be a great book to have on the shelf in the shop!).

[unclej]

Yeah, I wasn't questioning your judgement or anything, just wondering about what lead you to that conclusion, and if I was missing something. Someday we'll have to put everybody's notes together and write the quintessential troubleshooting manual (maybe we could get RG to edit - now that would be a great book to have on the shelf in the shop!). 

←

i'd buy a copy!

[1guitarslinger]

Now I get it...

[Paul Marossy]

Yeah, I wasn't questioning your judgement or anything, just wondering about what lead you to that conclusion, and if I was missing something.

No worries lk. I am willing to bet that particular amp is more likely to use a power tube bias tremolo than the Fender approach.

Someday we'll have to put everybody's notes together and write the quintessential troubleshooting manual (maybe we could get RG to edit - now that would be a great book to have on the shelf in the shop!).

Yeah, I guess that could be a kind of expanded "Tube Amp Debug Page". There are books out there that are supposed to be pretty good, though.

[h20rider]

Hey Guys, Thanks for all the replies and help.

The model amp is:

HARMONY 306-A (two channel with trem), one 12" speaker

Looks to be 6 tubes...all labeled "Japan":

(Two) 7025 12 AX7 These are shielded (removable shields)

(Two) 6V6

(One) 6SH7 Encased in metal

(One) 5Y3

The five I can see all light up. There is no change in any when I turn the 2 tremelo knobs.

All the connections look intact. No obviously broken resistors or capacitors.

I did change out the potentially lethal ungrounded powercord for a grounded one and grounded it to the chassis.

I'll take the tubes for testing.

Any more suggestions greatly appreciated.

[1guitarslinger]

You may be able to obtain a schematic for that amp through this site:

Harmony

At this point, testing the tubes is the thing to do before anything else.

[lovekraft]

My best guess (and an educated guess is all it is) would be that the 6SH7 is the tremolo oscillator, and it needs replacing. That tube list implies a fairly common small amp configuration used occasionally by Valco, Silvertone, etc. - one split 12AX7 with an input for each triode, driving a 12AX7 PI into a pair of 6V6s, with a pentode phase-shift oscillator modulating the power tube bias for tremolo (usually a 6AU6). If you have access to a 'scope, it would be fairly simple to just check for a low frequency signal on the depth/strength pot to verify that the problem is there and not elsewhere. Beyond that, without a schematic, I'm out of ideas.

[1guitarslinger]

I agree LK, but I am currious after what Paul Marossy brought up. I tried to find a schem really quick to satisfy my curiousity, and hopefully point everyone to it, but no dice.

[lovekraft]

Yeah, I dug pretty deep too and couldn't come up with one - the basic setup sounds suspiciously like the old Silvertone 1472, but the rectifier and trem oscillator tubes are different. Might be time to call the Global Schematic Network!

[Paul Marossy]

I haven't been able to find anything, either. But, it sounds like lk is on the right track on this one.

This is what your amp looks like, right?

Edited October 7, 2005 by Paul Marossy

[Paul Marossy]

Found a schematic! It's not a scan, but a decent picture of it from a book.

http://www.diyguitarist.com/Schematics/Harmony306A.jpg

So, does this use a phase shift oscillator that modulates the power tube bias?

[1guitarslinger]

Found a schematic! It's not a scan, but a decent picture of it from a book.

http://www.diyguitarist.com/Schematics/Harmony306A.jpg

So, does this use a phase shift oscillator that modulates the power tube bias?

←

Well how about that. Good job finding the schem!

Yes, it looks like it does.

[lovekraft]

Paul, you rock! How cool is that!!?! Yes, that's almost exactly what I suspected - see how the plate of V5 is AC coupled to the power tube grids' bias point (point A) through C15, R33 and R35? So, all you have to do is turn R35 wide open and make sure the LFO signal is present (and strong) on the hot side of the jack - if it's there, and you're not getting tremolo, there's most likely a problem in the wiring going to the power tube grids, and if it's not, it's probably a fault in the oscillator circuit (V5, R27-35, C15-19 and the power connection to R31 and R32). Anyway, that's where I'd start poking around - if the LFO signal wasn't showing up at the jack, the next thing I'd check is the tube, and then the power connections.

I'll bet that thing sounds nice dimed out - I love the sound of a cathode biased push-pull pair running out of gas when they're maxed!

And don't forget - potentially lethal high voltages that may cause death, sterility, blindness, blah blah blah, etc.,... just be careful, OK.

Edited October 7, 2005 by lovekraft

[Paul Marossy]

Yeah, I bet that's actually a good sounding amp. I just got lucky finding the schematic, but that is pretty cool, isn't it?

[h20rider]

The box still has the schematic pasted inside. It's too old to remove and copy, but if there is any info there that I can forward, I'll study it. Again, I appreciate the interest and consideration you all are showing.

Gery

[h20rider]

Well, all the tubes check out fine...well into the "good" zone on the tester. I'll try some of the other suggestions now. And yes, Paul's posted picture is what mine looks like. I've only seen ocilliscopes in old sci-fi movies...so that one may be beyond me.

Thanks, Gery

[1guitarslinger]

And don't forget - potentially lethal high voltages that may cause death, sterility, blindness, blah blah blah, etc.,... just be careful, OK. 

Another thing that'll cause blindness? Oh man...

[Paul Marossy]

Well, all the tubes check out fine...well into the "good" zone on the tester.

It could still be a bad tube. If your tester doesn't test transconductance, but is only an emission tester, it may not be telling you the whole story. Best way to test that is swap it out with a known good working one. OTOH, it could be a cap that gave up the ghost...