Tubes problem?

[fuzzspiral II]

Hi, I didn't find a possible answer reading other threads so I'm asking here, feel free to redirect me on the right one if I'm redundant.
Up until now my SVT CL had no issues but the last two times it couldn't saturate as usual, at same eq and gain level just produced a "fizzy" sound instead of its gritty/driven saturation.
The amp was new when I bought it, seven years ago. Since than I always used it once or twice a week, gain at 7/8, heavy EQ on mids and lows, overdives pedals, ecc
I checked the bias both times and they were ok, could it be that the valves need to be changed? I noticed that they aren't "incandescent" as usual, the hot on the rear grill is still there after three hours of playing but the tubes seems more "off", not with the vivid orange and blue they always have been.
The cab speakers are ok, I tried with cleaner sounds at same volume and they sounded as they should.
Any ideas? Thank you

PS the drummer has the very bad habit to plug and unplug cables on the mixer (with only voice and to two active speakers connected) and inevitable "POPS" things sometimes happen and it's easier asking the two guitar players not to play solos than convincing him not to do it... Just saying this fot further informations

EDIT. In case of bad electrical system, a current filter that keeps the voltage stable could help?

 

[agedhorse]

You can’t tell if a tube is “worn out” or failing by looking at it.

It’s not bad power, don’t waste your money on filters.

It MAY be a tube that is failing, but it could be a lot of other things too.

 

[Wasnex]

could it be that the valves need to be changed?

Yes...but it could be a lot of other things as well. There is really not much you can do to trouble shoot the amp without the necessarily electronics background and test equipment. The least risky option is to take the amp to a qualified repair tech.


The SVT CL has a pretty comprehensive bias monitoring circuit for the output tubes (a sextet of 6550s). Basically the 6 output tubes are organized into two circuits of three tubes. Let's call the circuits A and B.

The CL also has two sets of bias controls and indicator LEDs. One side adjusts and monitors the three tubes in circuit A and the other side adjusts and monitors the three tubes in circuit B.

The green bias indicator LEDs uses AND logic. To get green all three tubes in a circuit need to pull 15mA or more.

The red bias indicator LEDs uses OR logic. If one or more tubes in a circuit pulls 30mA or more, you get red.

If you can adjust for green without getting red, it means all three tubes are operating and matched within 50% or better. The more range you can adjust for green without getting red, the closer the matching.

If you get red before green, then tube matching is not within 50%. I.E. at least one tube pulls >30ma before the other tubes pull >15mA.

If you only get red and never green, one or more tubes is probably dead or nonop.

Poorly matched or dead tubes will reduce power and possibly cause hum. It's possible the output tubes are really worn and weak, but still matched fairly closely.


The preamp tubes are self biased.

As a user this is about all you can do.

1. Fiddle with the controls to try and identify where the problem starts. (see block diagram below)
   1. If you know how to read schematics, the preamp and power amp schematics are here: https://www.thetubestore.com/lib/th...5-DcyAiRMCW4iSIjcvgTYIqDXdzCKWSV3kWjpFA-I_kFm
2. Run a short shielded jumper between the Preamp Out and Power Amp In. The Power Amp In jack has a switching contact that can cause this sort of problem. The jumper bypasses the switching
3. If you are not technically minded, take the amp to a qualified repair tech.
4. If you are technically minded:
   The following requires partially disassembling the amp which exposes you to potentially deadly voltage and scalding hot tubes. Make sure you understand and know how to mitigate these hazards. Otherwise take the amp to a qualified repair tech.
   1. Reseat all of the tubes. Tubes can work there way out of the tube sockets over time.
   2. Roll a new 12AX7 through the amp. There are two 12AX7s in the preamp and one 12AX7 in the output section. Replace one tube at a time.
   3. The amp also has a pair of 12AU7 driver tubes in the output section.
      1. The drivers control bias in the output tubes; so before replacing a 12AU7, I suggest setting the bias for minimum tube current in the output section. After installing a new 12AU7, perform the bias procedure in the manual. EXPLANATION: A new 12AU7 may pull significantly more current than an old 12AU7. Increasing current in the drivers drops the bias on the output tubes and may allow them to pull excessive current.
   4. If the problems persists, take the amp to a qualified repair tech.

 

[eniac]

Since your signal chain is <bass guitar> -> <effects> -> amp, the first place I'd look is a defective effects unit, or maybe a defective power supply powering those effects, or maybe even a defective patch cable.

Seems to me you've got less than 6,000 hours on the amp total, worst case. If the output tubes can be biased properly, then it's unlikely those are weak, unless there's something wrong with the biasing circuit or how it's displaying the results. Also you say if you play the amp clean, things sound fine, so that too points to a problem someplace other than the power amp stage. As for the preamp tubes, you should get years more life out of those unless one of them was marginal when the amp was assembled at the factory.

 

[Wasnex]

Since your signal chain is <bass guitar> -> <effects> -> amp, the first place I'd look is a defective effects unit, or maybe a defective power supply powering those effects, or maybe even a defective patch cable.

Seems to me you've got less than 6,000 hours on the amp total, worst case. If the output tubes can be biased properly, then it's unlikely those are weak, unless there's something wrong with the biasing circuit or how it's displaying the results. Also you say if you play the amp clean, things sound fine, so that too points to a problem someplace other than the power amp stage. As for the preamp tubes, you should get years more life out of those unless one of them was marginal when the amp was assembled at the factory.

I would suggest assessing the amp with the bass plugged straight in...no effects or other pedals.

Make sure the instrument cable is good. Make sure the bass is good. If the bass is active, make sure the battery is good. Make sure the speaker cable is good. Make sure the speaker is good.

 

[eniac]

I forgot to mention the following. However, these have a very low probability of causing the problem described, especially since the amp sounds fine played "clean". But they're easy tests with zero potential for damage.

1. Connect Preamp Out to Power Amp In with a shielded cable (instrument cable, patch cable, or similar) and see if things improve.

2. Make sure the impedance selector switch is set correctly. You might even want to slide it back and forth a couple of times (with the amp OFF!) in case corrosion has formed on its contacts. (I've never heard of this happening, though. Still, it costs you nothing to try, except for 2 minutes of wasted time.)

 

[fuzzspiral II]

Thank you all for the tips, I'll try all of them in a couple of days (@Wasnex thanks for the schematics but I don't understand them...).
I could have thought about the cables, as a friend of mine also told me but we were concentrated on preparing a gig
I'll let you know!

 

[beans-on-toast]

From Ampeg:

Any time you notice a change in your amplifier's performance, check the tubes first. If it's been a while since the tubes were replaced and the sound from your amplifier lacks Punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced. If your amplifier squeals, makes noise [ed. including crackling, popping, or hissing, frying bacon], loses gain, starts to Hum, lacks "Sensitivity", or feels as if it is working against you, the preamplifier tubes may need to be replaced. The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently, they almost always fail/degrade first. If deteriorating power tubes aren't replaced they will ultimately fail. Depending on the failure mode, they may even cause severe damage to the audio Output transformer and/or other components in the amplifier. Replacing the tubes before they fail completely has the potential to save you time, money and unwanted trouble. Since power tubes work together in an amplifier, it is crucial that they (if there is more than one) be replaced by a matched set. If you're on the road a lot, we recommend that you carry a spare matched set of replacement power tubes and their associated driver tubes. After turning off the power and disconnecting the amplifier from the power source, carefully check the tubes (in Bright light) for cracks or white spots inside the glass or any other apparent damage. Then, with the power on, view the tubes in a Dark room. Look for preamplifier tubes that do not glow at all or power tubes that glow excessively red.

Groove Tubes: some tubes may need to be changed if your amp is experiencing any of the following symptoms:

loss of tone, clarity, Sustain and harmonic richness
inconsistent Output level
lack of mid-range Punch and Definition
rattling, whistling or humming
feedback or metallic sound on certain notes
Weak sound and power loss.

 

[agedhorse]

From Ampeg:

Any time you notice a change in your amplifier's performance, check the tubes first. If it's been a while since the tubes were replaced and the sound from your amplifier lacks Punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced. If your amplifier squeals, makes noise [ed. including crackling, popping, or hissing, frying bacon], loses gain, starts to Hum, lacks "Sensitivity", or feels as if it is working against you, the preamplifier tubes may need to be replaced. The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently, they almost always fail/degrade first. If deteriorating power tubes aren't replaced they will ultimately fail. Depending on the failure mode, they may even cause severe damage to the audio Output transformer and/or other components in the amplifier. Replacing the tubes before they fail completely has the potential to save you time, money and unwanted trouble. Since power tubes work together in an amplifier, it is crucial that they (if there is more than one) be replaced by a matched set. If you're on the road a lot, we recommend that you carry a spare matched set of replacement power tubes and their associated driver tubes. After turning off the power and disconnecting the amplifier from the power source, carefully check the tubes (in Bright light) for cracks or white spots inside the glass or any other apparent damage. Then, with the power on, view the tubes in a Dark room. Look for preamplifier tubes that do not glow at all or power tubes that glow excessively red.

Groove Tubes: some tubes may need to be changed if your amp is experiencing any of the following symptoms:

loss of tone, clarity, Sustain and harmonic richness
inconsistent Output level
lack of mid-range Punch and Definition
rattling, whistling or humming
feedback or metallic sound on certain notes
Weak sound and power loss.

I would replace "probably" with "possibly", or "maybe".

 

[beans-on-toast]

I would replace "probably" with "possibly", or "maybe".

Possibly maybe. But as you said, there are many other possibilities. For most players, swapping out the small signal tubes, one at a time with known good ones, is a good thing to try before resorting to a tech.

Unfortunately, for the CL, this involves some disassembly and partially sliding out the preamp chassis. A big job. Two of the tubes are in the pre-amp chassis.

 

[eniac]

This problem apparently occurred all at once. It wasn't something that showed up gradually. So I guess I should ask whether the amp was bumped hard just before the problem appeared, and if so, whether the tubes were hot at the time.

I'm still putting my $$ on what I suggested in #4, though. Mostly because the amp sounds completely normal played clean.

And something else... The effects are located before the instrument input, not spliced into the preamp out / power amp in jacks, correct?

 

[fuzzspiral II]

No it wasn't bumped, I always keep the amp on the cab, even waiting for it to cool down before closing with the case to avoid condense (a lot of humidity there...)
Yes, the effects are all before the instrument input

 

[fuzzspiral II]

Hi! All is functioning properly. I changed cables, no pedalborad, then I used the same cables again, pedals, ecc
In all these cases the sound and the gain worked as they should, go figure!
The only difference from before is that I changed the strings before playing, after a "what if" came into my mind, you know, "just in case"... (yes I am one of those guys )

 

[agedhorse]

This is why I always recommend basic troubleshooting before jumping to ridiculous conclusions.

 

[fuzzspiral II]

Thanks to all for the support