AC15HTTVH farty and fizzy...

[hebaton]

Hello, I am not a builder but I have made simple mods to a couple amps based on minimum knowlage and directions found on the Rob Robinette site.
I recently aquiered a VOX AC15 HTTVH and I am having trouble understanding what I get. I find the amp gets completely overwhelmed with humbucker pickups. There is way to much bass and everything falls apart as soon as the volume is past 2 or 3 ( top boost channel ) I find the overdrive tone I get when pushing the amp is fizzy and unpleasant. just "on the edge" settings can work but anything past is bad news.
THe EF86 channel seems to work somewat better but the current tube is noisy making a loud Hiss. I assume this is "part of the deal" but I think this one is past its best days.

CAn bad output tubes behave like that ?

All that said, I find great clean tones with both humbuckers and single coils.. maybe SC sound better...

My impression is that the thing needs a full retube, the bigt cap needs to go and maybe there are things that can be done to make it more buckers friendly ?

ANy suggestion will be appreciated. To start with, help me identify the bright cap on this thing please ?

Thanks !

[xtian]

AC15s and AC30s are very hard on power tubes, and the symptom you describe could easily be in the power amp. First thing is to check the screen and grid resistors, then put in new power tubes. Preamp tubes are much less likely to fail.

[hebaton]

THank you,
I have tubes on order and i'll check what those resistor ead like. I keep hearing the bright cap on those has to go. make sense ?

[xtian]

I keep hearing the bright cap on those has to go. make sense ?

Entirely a subjective matter of preference. Won't cause the issue you describe.

[pdf64]

It’s a good idea to get a newly acquired amp checked over by a competent person to verify that it’s electrically safe and is working correctly.
If you’re sure it’s ok, perhaps it’s had mods undertaken?

[ChopSauce]

Modded (could have said damaged) up to that point? I can hardly believe it. If that is, it is likely to show on pictures of the circuit, if you can post some.

Also ... when talking about humbuckers, what kind: high output ones?

If the circuit had been modded to vintage specs, with bigger (~47nF) coupling caps on the first stages, it can be overwhelmed by those kind of "modern" ceramic pickups. At least I could do that with Epiphone ones from the '90s into a vintage AC4 clone - but the fizzy thing, maybe...

[hebaton]

Here are some pics of the pream section. I need to remove the bottom plate if I want to acces the Power section.
I am assumn the large caps on the inputs are the "bright caps" correct me if I am wrong please.

When I say humbuckers, I am talking 8K and 8.5 K output.
THe head is pluged into a single 12 matching CAB with an alnico blue.

[hebaton]

I can also add that running the power section in triod mode significantly reduces the problem.
Maybe this is to be expected but it seems to me the issue is with the output stage or am-I getting this all wrong

Also, it was suggested to change the C5 and C6 values ( output copling ) to .022uf I have 2000 V on hand. Would change the over all tone of the amp or just make it less sensible to strong input signlas ?

FInally, I also read about changing the phase inverter bias resister from 1K2 to 820-900
I have this ( see pic ) on hand that reads 810 ohms.

Someone please weigh in ?

[Colossal]

I can also add that running the power section in triod mode significantly reduces the problem.
Maybe this is to be expected but it seems to me the issue is with the output stage or am-I getting this all wrong

Also, it was suggested to change the C5 and C6 values ( output copling ) to .022uf I have 2000 V on hand. Would change the over all tone of the amp or just make it less sensible to strong input signlas ?

FInally, I also read about changing the phase inverter bias resister from 1K2 to 820-900
I have this ( see pic ) on hand that reads 810 ohms.

Someone please weigh in ?

This is a used amp, right? An AC15 does not have much headroom with humbucker guitars. If this is a used amp, and it has not been gone through by a tech to ensure that it is working as intended, I think it's important to rule out that what you are experiencing is either aged power tubes, tired power supply capacitors or possibly some other issue (if there is an issue..), or that it is just the voicing you don't care for.

You could certainly swap that 1k2 phase inverter tail resistor for your on-hand 810R. This could be an interesting test, because that would add more gain and a little more warmth, and bring it towards the Marshall PI voicing.

[hebaton]

Yes it is a used amp, built in 2007 I beleive.
Here is the schematic I am refering to. I think it is the corect one.
IF it is, then the resistor in question would be R15 right ?

I did not have it verified. no one anywhere near where I am....
But I do have tubes on order. power tubes for now. DO rectifiers in those often need replacement ?
and could you please, if you have a miniute for this, identif those power supply caps for me ?

Thanks again !

[hebaton]

I did the 810R on the PI. Jury still out in that. I did not want a Marshall. Perhaps something in between, thinking 1K...
Anyway I will now wait untlI can replace the tubes to make any other change. I do however, still welcome your input, all of you !

[Colossal]

if you have a minute for this, identify those power supply caps for me ?

C1 22uF B+ C-L-C
C21 22uF B+1 (Power tubes/output transformer)
C2 10uF B+2 (12AX7 Phase Inv)
C3 10uF B+3 (EF86)
C17 10uF B+4 (12AX7/ECC83 Top Boost second gain stage + cathode follower)
C18 10uF B+5 (12AX7/ECC83 Top Boost input)

[ChopSauce]

It doesn't look modified at all, at least. I hope that changing the power tubes will solve your trouble.

[jabguit]

When I say humbuckers, I am talking 8K and 8.5 K output.

8K and 8.5K is not ouptut - it's resistance.

cheers,

[hebaton]

When I say humbuckers, I am talking 8K and 8.5 K output.

8K and 8.5K is not ouptut - it's resistance.

cheers,

Thank you. I know but, is that not how pickups are generally refered to ?