Tube Depot 5E3 feed back loop jumper question.

[Mungo Park]

Hello All, first post, wonderfull forum you all have here.
I have bought an alredy assembled tube depot 5e3 kit. It was a good price I could not resist. And figured it would be good to learn more about the 5e3 and the many mods.
In reading up on the kit it has a jumper on the board that when in place gives a feeback loop. I was of the understanding it can be removed to give more of the original 5e3 sound. I figured I would try it without the feedback loop and see if I wanted to put in a switch to have both as a option.
So I took it out, the amp gave a extreme distortion sound with my guitar and some poping and clicking for maybe 20-30 seconds. It then produced a normal rather sweet sound with maybe a slight reduced volume, but a ckick with the string attack. I shut it down and put the jumper back, figuring I was lucky I did not fry the amp.
Any ideas why this would act this way and not function well.
Cheers Ron.

[Jazz P Bass]

First of all, who built the kit.

Do you have any documentation showing this 'jumper'?

Here is a shot of one way to connect NFB on a 5E3.



A wire will come off of the OT secondary & go through a resistor to the preamp.

Please tell us if yours is the similar & where is the jumper located.

[Mungo Park]

Here is the layout:
https://d1sjrnpi226dnf.cloudfront.ne...pdf?1400188816

And here is the schematic:
https://d1sjrnpi226dnf.cloudfront.ne...pdf?1400188790

When I bought the amp I thought the seller had built it but no, so the builder is unknown. It does sound nice as is but when I bought it I figured I could explor some mods.

I can not really tell from looking at the board as it is bolted in with all the traces on the back side. I hope the above links will help.
cheers Ron

[The Dude]

Welcome to the place! Pretty sure I got it, but to be clear, you removed J1 jumper which removed NFB. If that is the case, I can't think of a good reason for your problem. With the jumper removed it is the same as the original 5E3, which has no NFB. Here's a link to RG's debugging page, which may help you.

Tube Amplifier Debugging Page

[Mungo Park]

Thanks for the warm welcom, squinting at the schematic it states removing the jumper switches the amp from cathode bias to fixed bias with the jumper out.
Does this sound diferent than feed back loop, to me this is more how the tubes are biased? Maybe the feedback loop was mis information, well relative to the jumper. Still the amp should work both ways.
Cheers Ron.

[The Dude]

Ok. That's why I asked. Jumper J1 can be removed all together to remove NFB. The jumper you are talking about is 'G5' to 'aa'. THIS JUMPER CANNOT SIMPLY BE REMOVED! Options are 1) leaving in the jumper for cathode bias OR 2) tie point 'aa' to negative bias for fixed bias.

[Mungo Park]

Thank you for taking the time to help me out with this, I guess the option would have to be built in as part of the orginal assembely.
Thanks again cheers Ron.

[The Dude]

Yes. The kit, as is, has no negative supply. You would be talking about more modification. If you look at the square at the bottom of the schematic, it shows a negative supply that would need to be built on "a separate terminal strip". The builder of your kit didn't likely build that part, although it's certainly a possibility.

Edit: Also, if you look, you would install the 1 ohm resistor from the output tube cathodes to ground for the fixed bias option effectively bypassing the existing cathode resistor and cap.

[Jazz P Bass]

Sooo.

Leaving jumper G5 installed (cathode biased), how does the amp sound when you remove J1 (NFB)?

[Mungo Park]

So...Yes good eye, I guess that was my intent but removed the wrong thing. I will give it a try tonight and see how it sounds.
Thanks for your insight, and patients.
Cheers Ron.

[Jazz P Bass]

Alright.
To recap.

When you removed G5, you also removed the ground reference (through the two 220K resistors R17/18) for the grid resistors (R16/19).
That is what all the noise was about.

For a fixed bias output section, there is typically a negative voltage applied to point "aa". (assuming G5 is removed).
That negative voltage is what sets the idle operating point of the tubes.
By installing the 1 ohm resistor in parallel with the cathode resistor (R20) the effective resistance (250 & 1 ohm in parallel) is still 1 ohm (well a few hundredths lower).
So with the cathode resistor out of the picture, the 1 ohm resistor sets up the cathode for fixed bias operation.
Usually the cathode is at ground.
By installing a 1 ohm precision resistor you have the added benefit of a test point for measuring idle current.

[eschertron]

So...Yes good eye, I guess that was my intent but removed the wrong thing. I will give it a try tonight and see how it sounds.
Thanks for your insight, and patients.
Cheers Ron.

Removing the GG - aa wire and 'losing' the bias may have done something bad to the output tubes. Cross your fingers and if there's anything odd about the amp, with jumper back in place, turn it off immediately! Odd can mean heat, smell, loud noise, no sound among other indications.

As JazzP said, removing jumper J1 can be done at any time without trouble.

...and welcome!

[Mungo Park]

I have put the G5-aa jumper and tried the amp again. it sounds the same as original, so fortunately no harm, fingers crossed.
jazzP I have printed the schematic and drawing out in red what you have noted. Thank you for the detailed explanation.
I will get my head around this part first. Then look at the J1.
Cheers Ron

[Jazz P Bass]

I would not mess around with the fixed bias option.

Cathode bias is good enough for a 5E3.

If you want to study it, the bias tap on the transformer secondary is probably taped off.
On the schematic, upperleft is a physical drawing of the supply.
Lower right, is the supply itself. (a negative voltage. Notice the diode is reversed)

tweed5e3_Fixed Bias Supply.pdf

And so that you will be totally confused, they where kind enough to include a Marshall type bias supply. (lower left)

[Mungo Park]

Oh I was not going to mess with the bias, just wanting to absorb it to understand how it all goes.

Cheers Ron.