Clean channel getting quieter

[PhatPhish]

Hello all.
I have an HT5-R which, although it's a few years old now, has not had a very hard life and has been serving me well for home use. However, it's started behaving a bit oddly, so I thought I see if anyone here had any ideas what the cause of the problem might be.
I tend to use the OD channel mostly, but I've noticed that when the amp's been on for a while (say more than half an hour) when I switch back to the clean channel the clean volume is a lot less than previously. The next day, it's back to normal....but then gets quiet again after the amp's been on for a while.
Otherwise all seems fine - no problems with volume on the OD channel, no nasty snaps, crackles or pops, and sound is fine....just diminishing volume on clean channel.
Has anybody got any ideas or suggestions please?
Thanks

[thephantum]

Diminishing volume is usually a symptom of a failing tube. It's not really a matter of how hard a life the amp has had...it's more about total length of time the amp has been powered on as well as the number of power/heat cycles. Since you've had it for a few years, it might be time for a re-tube.

I would start with the preamp tube (12AX7). Try swapping it for a known good one. If that fixes it, you're done. No biasing is necessary.

If that doesn't work then try swapping the power tube. If that fixes it then it needs to go to a tech for biasing. If that doesn't fix it then it needs to go to a tech for further diagnosis.

[PhatPhish]

Thanks for your reply...the power/heat cycles thing makes complete sense.

I thought that you only need to bias if you have valves in sets. My understanding is that the HT5-R has a single pre-amp valve and a single power amp valve, so could I change these myself without the need for biasing? Sorry if that's a daft question, but all the stuff I've read online today about biasing seem to talk about matching pairs or sets of valves.

[thephantum]

Long story short is that the HT-5 is a Class AB, Push-Pull amp and needs to be biased.

You are correct: One output tube usually means that it's a single-ended, Class A amp. Amps of this type do not normally need to be biased (which is why you can normally swap preamp tubes without biasing....most guitar preamps are Class A). But this is where looks can be deceiving. The power tube in the HT5 is a 12BH7A, which is a dual triode tube or two separate amplification stages in one tube. The HT-5 leverages those two triodes as two different tubes in a Class AB, Push-Pull configuration. It therefore needs to be biased.

This is further evidenced if you pull the chassis out. There are two trim pots right next to each other labeled BIAS and BAL. The first sets the overall Bias current, the second sets the balance between the two triodes...allowing you to adjust for a tube with slightly mismatched triodes.

[PhatPhish]

Great info - thanks again