Adapted 5F2A with EZ80 and EL84

[stephenhartley]

Understood. However, that is a different issue than what I was discussion in post #27. If the buzz you mentioned stops when you plug in the guitar then I don't believe it is caused by the grounding topology that you just described. Are you using shorting jacks for your inputs? If so then the tip shunt switch contact should be grounded so that the input is shorted to ground when you unplug the guitar cord. I'm still wondering what type of input jacks you are using and how they are wired.

Thanks - I'm using Switchcraft switched jacks. I've just copied the Fender wiring on the layout, but I'll check it again.

I'm beginning to think that I should just change the PT - I struggled to find one in the uk. The Hammond one is a huge hefty thing that's big enough for a 5E3 and has a 5v tap which I don't need for an EZ80.

[Chuck H]

Here's a copy of the 5f2a schematic altered to show the changes I would suggest in red. I know it was suggested that the nfb be increased. I disagree because the harsh tone in these little SE amps is usually symptomatic of other issues and (IMHO) el84's just sound better with a little LESS nfb. In light of your consideration of changing the PT it should be noted that the change to the HV rail resistor feeding the preamp tube was considered with the PT you're using now.

EDIT: It seems the forum will not let me attach files at this time. Will explore other avenues.

EDIT 2: Ok... No way to attach images from your PC anymore. It was already hinky, but now it's actually gone. Sorry.

[stephenhartley]

Thank you so much for your effort Chuck.

I abandoned ship last night and swapped the PT for a Hammond 291AEX using the 550 tap (it reads 588).

I won't have time to work on it for a couple of days.

I would love to see the schematic - I'll see if I can message you.

Tried to upload some pics but doesn't seem to work.

[Dave H]

Here's a copy of the 5f2a schematic altered to show the changes I would suggest in red. I know it was suggested that the nfb be increased. I disagree because the harsh tone in these little SE amps is usually symptomatic of other issues and (IMHO) el84's just sound better with a little LESS nfb.

The suggestion was to increase (double) the feedback resistor value to decrease the nfb



If you can see the above attachment pm the marked up one to me and I'll upload it.

[Chuck H]

Ah! Ok. Guess I read through that too fast... Good suggestion

[stephenhartley]

We finally got our 5w 5F2A/EL84 amp sorted and it sounds awesome - and very loud! I'd just like to thank everyone for their help.

With the 275-0-275 PT, the plate voltage is 333v - a cathode resistor of 250R is giving a dissipation of around 13W.

The break-up into brittle distortion at 2 was sorted with a grid-stopper resistor of 100R.

The hum is low and the tone is quite responsive.

[Tom Phillips]

Thanks for reporting the outcome and I’m glad you got the amp sorted out. It is interesting that a such a low value (100Ω) grid stopper resistor would make such a significant improvement.

What was the fix for the excessive buzz that you reported when no guitar was connected to the input?

[g1]

Also, the 100R grid stop was control grid and not screen grid, correct?

[Chuck H]

Right! I'm thinking it's a screen grid resistor and not a control grid stopper resistor. It was sort of hinted at earlier.