5E3 based amp

[stephenhartley]

This is my 5E3 based amp (if the youtube link below works). Thanks to all on here who have given help and advice. I've got the instrument channel on 2 and a half. I have the amp biased cool at 22mA with a 500R resistor and I like the clean sound (I use a distortion pedal if I need to get a good dirty sound at low volume). Unfortunately the mic channel has quite a bit of hum and I don't know what to do next to get rid of it. The buzzing on the video by the way is the camera not the amp. The video is recorded in widescreen, but YouTube has squashed it. The amp is actually roughly square. I'm using my Gretsch Tennessean. The tune is written by Jesse Spence and I've tried to copy the Ry Cooder version. The top two speakers are 6" 60s Wharfedales and the bottom 2 are Jensen alnico P8s - with distortion turned up full on my pedal, the speakers give out an awesome cyclical screaming feedback - my son tried it out with his Strat last night and it sounded great.

Great Dream Fom Heaven by The Reverend Jesse Spence played by Stephen Hartley - YouTube

[MWJB]

What is different about the grounding & wire routing of the mic channel compared to the instrument channel? Posting some pics of the V1 tube socket wiring, jack & pot wiring and a description of you grounding scheme might help.

[stephenhartley]

I think the problem might be due to the chassis layout because it's upside down, so the input sockets are at the opposite corner of the chassis to V1. I've experimented for hours tonight and I can't get rid of the hum on the mic channel. I've moved the 2 68k resistors to the mic channels sockets with a shielded cable to V1. If I ground the V1 end of the screen at the V1 end, then there is no sound from the channel, but it works if I ground the shield to the socket end. With neither end grounded, there is a loud buzz. The shielded cable from the tone circuit to V2 is grounded at both ends. If I lift the ground at the V2 end, then there is no sound from the amp at all. The chassis is made from 2 channels and a flat plate. I have run earth straps from the channels to the plate and most of the earths are to one of the PT bolts and to one of the OT bolts at the other end. I'm puzzled why the instrument channel is OK. Pics aren't very clear.

[Chuck H]

Could you post a couple of full chassis 'each side half chassis' shots? It looks like you have some layout infractions in there. At least one biggie I can see.

[MWJB]

:-o!

"I think the problem might be due to the chassis layout because it's upside down, so the input sockets are at the opposite corner of the chassis to V1." I would say that is definitely part of the problem. That and the routing of just about every wire.

Salvage the parts you can & buy a proper chassis...a tried & tested circuit board wouldn't go a miss either (reduce your margins for error) Inputs & preamp together at one end, power transformer rectifier & power tubes at the other. Give up on this chassis, it will NEVER work properly. You said that you are in the UK, if you want some tips on layout feel free to PM me & I'll get round to it when I can & when you have a chassis.

[stephenhartley]

Thanks - I've learnt a tremendous amount from building this amp i.e. how not to do it. It's my first foray into proper electronics. The whole point for me is making everything myself in my small back room. Despite the chassis layout being all back to front, the amp actually sounds great - the hum on the mic channel is a bit on the loud side, but I can live with it.

I'll make sure I use a correct layout for the next one. Thanks again for your advice.