Express hum on full volume. Normal?

[Littlewyan]

My Express sounds great with all controls set at 5 but if I whack the volume up to 10 there is a lot of hum. Its especially noisy if every control is at 10. I know the Express is obviously a high gain amp but is it normal for it to hum with the volume on 10?

I think the hum is driving the output stage hard as well as with the volume on full the output power actually drops by a few watts.

[Colossal]

I don't think you should have hum in the amp at any volume, just hiss, and that can be minimized with preamp tube selection. Wondering if it's pickup hum you are getting or proximity of your guitar to the power transformer. That amp is an antenna.

[Littlewyan]

I'm not ignoring you on this, just haven't had a chance to test the Express. Will do some valve rolling first to see if the hum changes at all.

[Littlewyan]

Actually its the valves! Swapped pre amp valves around and the noise changes. Some of them create a static type sound, others just more hum and some are quiet. Tried a Mullard in there and as it has so much high end I couldn't have the volume on 10 within putting the treble and presence down to 3!

I might consider adding small grid stoppers to make valve swapping easier.

[Littlewyan]

I just found that without the attenuator it howls if the treble or presence is too high. Different pre amp valves affect it. Tried grid stoppers and tried putting the amp on the floor behind the speaker cabinet but still same issue. I suspect its a combination of my V1 and V2 aren't good enough. V2 causes a buzz when put into V1 and V1 hums when the volume is turned up.

Looks like I'll have to go 12AX7 shopping.

Interestingly 2 JJ ECC83S and 1 JJ ECC83MG all buzz the same way in V1.

[Ron Worley]

The Express is arguably one of the most sensitive amps to the quality of V1. IIRC, Ken F favored Eastern European Tungsrams for his amps.

[Littlewyan]

I'm sticking to current production valves for mine but am struggling I must admit!

I get feedback without a guitar plugged in. So it can't be the first stage causing the feedback, must be the 2nd. I did try a 820R grid stopper here but it just made it more prone to picking up noise more than actually stopping the microphonics.

One thing I did find is the JJ ECC83 MG I have in V2 will buzz in V1, but will also buzz in V2 with certain valves in V1 :S. My V1 in this amp is just over a year old now and has been pulled in and out of this amp many times so I've probably made it a bit microphonic by doing that! In fact I don't own any 12AX7s that are in boxes unused so I'll have to order some.

If I remember rightly Ken said the Komet amps were designed for modern valves and if you look at them he has a 33K stopper on V1A and shielded wire on V1B. I wonder if thats what he had to do to get it quiet with modern valves.

[Reeltarded]

Your stopper is way too small. Try 33 on first and 68 on second..

Ks..

It's not even near the audible range of the guitar, your amp, the speaker, your cable. Your dog woud not notice if he had an ear horn.

You keep saying can't about all kinds of cans. You will be chasing the impossible. That's how it works.

[M Fowler]

I've found out from building many different types of amps that using an isolated input jack and separating out the filter cap grounds really works well in TW type amps.

PI filtering, B+1 and B+2 caps all going to a ground point by itself near the PT along with CT if you have one.

The filter caps for the preamp ground near the input jack.

Tubes can make or break an amp noise wise. Tube roll

Filament imbalance try a humdinger to balance the line, 2nd resort add elevated dc if cathode bias PS and final use regulated dc filament supply.

[Littlewyan]

I don't think a 33K stopper will make any difference on the first stage as the amp squeals when there is nothing plugged in. 2nd stage I can see it making a difference, but I was going to try a few more valves first to see if I can get it to be quiet.

Don't forget the pre amp valves in there are over a year old (with the exception of one) and have been pulled in and out of that amp so many times. I wasn't overly pleased about my Mullard being the most microphonic valve out of the lot though!

I did seperate my grounds when I rebuilt the amp Mark but unfortunately there is only so much I can do as I used cap cans for the filters.

Edit: Just stuck a JJ ECC803S in V1 for a laugh as I know being a long plate valve it will be much more prone to microphonics and rather embarassingly for the EH12AX7 it was much better and had much less hum!

[Littlewyan]

Sorted. I had an EH12AX7 in V1 and V3, swapped them around and now I can turn every control up to 10 no problem. Still hums a bit and buzzes slightly so going to order new valves still. The Express is actually pretty awesome with every control on 10. Very tricky beast to play at that level though!