Vintage AC10 (w/Trem no Reverb) ticking

[rooster]

So this is rather strange to my experience, the EF86 tube (a single 9 pin pentode) into the Pentode side of the 6U8A tube for gain and trem. The Speed aspect of the trem circuit is located within the pentode half of this 6U8A (ECF82) tube in this model and actually, if you've never played one, you will find that the trem itself can actually offer overall gain to the Trem (labeled 'Vibrato') Channel. (I will add, the triode stage of the 6U8A powers the 'Normal' channel (non-trem) and at full volume, with the trem engaged in the Vibrato channel (it's volume at ZERO), you can hear the pulse of the trem and a very slight bleed of the ticking. Still nothing like the tick volume you can hear from the 'Vibrato' channel.)

But there's a problem. The amp is stock, and the two electrolytic cathode caps (25uf) have been replaced. All the original coupling caps are in place (400vdc Philips Mustards) and the amp pretty much rips with a freshly broken in (Variac 20hrs) pair of Weber Silver Bells (2/10 model)....but with a rather annoying ticking happening if you engage the trem. The ticking (oscillation) is coming from the Speed section of the amp, the 6AU8 pentode half of the tube. (I will add, the triode stage of the 6U8A powers the 'Normal' channel (non-trem) and at full volume, with the trem engaged in the Vibrato channel (it's volume at ZERO), you can hear the pulse of the trem and a very slight bleed of the ticking. Still nothing like the tick volume you can hear from the 'Vibrato' channel.)

Q: Has anybody here run into this issue with this particular amp and did you find a fix for it? The 6U8A tube is a fresh NOS RCA version - and/but - I don't have a duplicate version to replace it with. Is the tube generating this ticking? Perhaps, but what I'm looking for right now is some input from someone who actually has this vintage amp. I want to know if they have experienced this issue and if they did, did they find a fix for it. Thanks.

ac10_2.pdf

[Reeltarded]

Steve is going to come along and describe a diode mod over the tickalo circuit or if you need to you can search the hoffman forums and fix it in 10 minutes.

look in the harmonic vibrato discussions we have had recently in the fender forum for links and fixes to various things related.

There is a thing anout diodes over the intensity for bias based circuits. I can't recall values and never used it.

I have a Domino/Vox AC15 with a 2x10 cab. Sounds like tiny Brian May with a Rangemaster in front.

[rooster]

Sweet! I have not heard about this diode trem fix in a fixed cathode bias amp so I am rather excited... I'll go look at the Hoffman forum right now and see what I can find. Thanks, Reeltarded!

Wait, do you mean the Hoffman Amp Forum?

[sluckey]

The diode trick is not applicable to the AC10 circuit.

[martin manning]

I would look at lead dress and make sure that the tremolo signal lead is well away from anything sensitive. Maybe consider trying a new cap at C14, and look for DC after C20 in the oscillator.

[Reeltarded]

nothing I say ever works

[pompeiisneaks]

Yeah the AC10 may be like the AC15/30 that uses a shift network and a tube oscillator. I'm not sure of the source of the ticking, except for maybe a bad tube in the circuit?

also, I am confused by the statement of the EF86 (both halves). That's a pentode and has only one 'half' no? Just trying to be sure I'm understanding what you mean.

~Phil

[rooster]

Oh, that's right, regarding the single function of the EF86, Phil. I'll correct that. Sorry guys, deep in here...

Yes, it's natural to say lead dress here - BUT - find me one documented post anywhere about any AC10...and then one with trem ticking issues. Not being over logical, but if this issue exists I would think someone would be talking about it. I find nothing. Is the amp this rare? As to replacing parts, I've swapped a few with no change. C14, no change, I will check @ C20 though. Thanks.

slucky, I appreciate your input here, your AC15 experience is solid. What do you know about the AC10?

[rooster]

Oh, I did want to talk about lead dress in this amp for another minute. Would you believe that the OT secondary ground is star grounded to the preamp ground? It is. And lifting it and grounding it to a more appropriate place on the chassis made no difference to the ticking. Also the bus bar, which is neatly suspended off the pots (but isolated from the Cut control), grounds not to the pots themselves but a ground point on the chassis, midway between the pots.

In so many ways I find this amp's layout a potential for oscillation and yet I will repeat: there are no conversations about this amp anywhere and (sadly) therefore, no comments about any issues.

[pompeiisneaks]

If I'm looking at the schematic right, it looks like the ECF82 Pentode side is the oscillator in that circuit. If it's having issues it could be causing the ticking. Otherwise the shift network could be not doing the phase shift perfectly and causing some kind of positive feedback/oscillation, make sure all of those 0.005uF caps are good, and well connected etc, and that the 1M to ground between them are still within tolerance etc. As for there not being no mention of this problem, I think your indication of there not being a lot of these is probably on point. I'm no vox expert but in the about 2-3 years I've been an amp nerd, I've loved Vox the most and have rarely heard of them unlike the 15, 30 50 and 100 versions. The schematic I'm looking at here seems to show the normal channel uses the tride side of that ECF82 no?

https://www.korguk.com/voxcircuits/circuits/ac10_1.jpg

~Phil

[sluckey]

I don't know anything about the AC10. Your trem circuit is totally different from the AC15 circuit.

I do have some experience with ticking trem issues in a preamp modulated circuit such as yours. So do most of the people that ever built a Marshall model 1974 18W trem amp. Just look at 18watt.com if they are still alive. Lot's of band aid type fixes but never was an "ah ha", definite fix that works for everyone. The circuit is just finiky and very sensitive to lead dress, wire length, layout, component placement, you name it. I found my band aid and moved on. Pretty sure it would not work for everyone and I doubt it will help you other than to say you probably have a finicky circuit too and your fix may be just as mysterious. Look at this schematic to see what I did...

http://sluckeyamps.com/18w/18w.pdf

[rooster]

Yes, you are seeing it correctly, Phil.

Hm, Phil, you mentioned parts being within spec? Well Q: Knowing that all CC resistors drift upwards - but (in this case) still deliver their function (resistance) - how can a working resistor create a tick in a working tremolo path? I have to think capacitor or layout. Bear with me. All of the Mustards are original. Perhaps 3 CCs have been replaced over the years (And perhaps trying to track the ticking down???). Do I want to be the guy to pull every resistor out of this AC10 that is not within spec because a capacitor is leaking? Or the layout is bad? No, I'm not going to be that guy. I am trying to investigate every possible idea of a fix before I replace anything. That's why I'm here, asking questions.

Slucky, thanks for your input and thoughts. I haven't built an 18 watt Marshall but the trem issue is interesting. I had no idea that particular trem circuit was prone to oscillation. Nice schematic, btw. I do own a Silertone Silver Medalist and it's trem is fine, using the 6AU6. In fact every Silvertone I've run into did not oscillate. Maybe this ECF82 is something to look at. I know nothing about this tube, only American counterparts, 6AN8 comes quickly to mind...

Martin, C20 shows 1-2mv DC dry (with trem off) @ pin 8, but obviously there is no way to measure leakage when the trem is engaged. Thoughts? What I'm hearing is seemingly a little more than what this small amount of leakage can produce.

[pompeiisneaks]

Yes, you are seeing it correctly, Phil.

Hm, Phil, you mentioned parts being within spec? Well Q: Knowing that all CC resistors drift upwards - but (in this case) still deliver their function (resistance) - how can a working resistor create a tick in a working tremolo path? I have to think capacitor or layout. Bear with me. All of the Mustards are original. Perhaps 3 CCs have been replaced over the years (And perhaps trying to track the ticking down???). Do I want to be the guy to pull every resistor out of this AC10 that is not within spec because a capacitor is leaking? Or the layout is bad? No, I'm not going to be that guy. I am trying to investigate every possible idea of a fix before I replace anything. That's why I'm here, asking questions.

Slucky, thanks for your input and thoughts. I haven't built an 18 watt Marshall but the trem issue is interesting. I had no idea that particular trem circuit was prone to oscillation. Nice schematic, btw. I do own a Silertone Silver Medalist and it's trem is fine, using the 6AU6. In fact every Silvertone I've run into did not oscillate. Maybe this ECF82 is something to look at. I know nothing about this tube, only American counterparts, 6AN8 comes quickly to mind...

Martin, C20 shows 1-2mv DC dry (with trem off) @ pin 8, but obviously there is no way to measure leakage when the trem is engaged. Thoughts? What I'm hearing is seemingly a little more than what this small amount of leakage can produce.

I'm just spitballing and could be wrong. The thought was that these phase shifts use the RC network to also perform some shunting to earth of a specific frequency no? If they get out of spec enough, it could create some weird frequency collision that's not expected... But, Again, I'm wildly guessing. Sluckey's tips are likely to point to a more useful answer, but even that seems tough.

~Phil

[rooster]

I saw this ad for an AC10 and a description. What remains of the original parts are the (2) Mustard caps on the two volume controls and the WIMA cap on the trem pot (far left). This approach, BTW, is called the 'shotgun technique' by Gerald Weber :

Rebuilt AC10 Panasonic caps and CF resistors.PNG

Description of work:

<<This AC-10 is a "Players" amp. It is not completely original. We had our amp tech go through this amp
and give it a complete service. All caps and resistors have been changed. These are parts that do wear
out eventually and have to be replaced. The power transformer has also been replaced. The 3 vent
covers on the top of the amp have been replaced. Power cord has been upgraded to a three prong.
I have taken some pictures of the inside of the amp chassis. (See pictures #9, #10, #11, #12).
The Choke and Output Transformers as well as all pots and of course the 10" Elac speakers are all original.>>

Now, it may be that this is what it will take to fix this AC10 that I have here but....

[rooster]

Note to all posters, this ticking problem was fixed with a new ECF82. It took a while to procure a new one but this took care of things. Out of spec resistors and vintage caps did not create the issue, and knowing it's tube inspired, it's quite possible a pf cap bridging the ECF82 input (pin8) to cathode might solve this issue with any tube. Fortunately, this wil not be investigated by me but to those having similar issues it might be something to look at.