Gibson EH160 - restoration advice needed

[mikeydee77]

I have in front of me a very pretty Gibson EH160 Amp (or so I believe). No idea on the year but it has a field coil and other signs of age but it is in very good condition.

This amp has not been powered up for some time but was reported working in the last 15yrs.

Observations...
- It has had some work with the inter-stage transformer replaced and a few electrolytics replaced.
- It has non-original 110v US 3 pin mains plug with the chasis connecteted to earth.
- Hot / Live goes to the fuse
- Neutral (as we call in UK) goes to effectively the zero star earth point where various electrolytics -ve meet.
- Green earth wire is connected to input jack earth.

Issues
When powering it up it blows my RCD but if I lift the earth (temporarily) then the amp will power up and does sound nice with a bit of hum in the background which could be improved no doubt.

My question is really about best practice on making this amp come close to modern safety standards and making sure it can be used without tripping the circuit breakers.

Additionally, I would like it to be converted to 240v operation but if the solution is to operate via an isolation transformer then that could solve two problems at once. Accordsing to the schematic, there is a 240v dropper resistor R8, which sits in a kind of rectifier socket, this one being 110 has a wirelink in there. Anybody know what the 100R 100w resistor would look like as that might be a cheap solution for 240v although sounds a bit dangerous.

Any other pearls of wisdom, history or help very welcome.

Mike

[Mick Bailey]

The only way this could be safe is to use an isolation transformer and properly earthed chassis - you can't run an amp that's in any way connected directly to the mains even with a dropper resistor.

[drewl]

Which can solve his 220volt problem.

[mikeydee77]

Thanks both I figured that was the way it was headed. Good to have reassurance of others.

Mike

[Jon Snell]

It goes without saying, fit an isolation transformer, 240 in and 110 out.
Isolation will make it safe after you remove the lower C4 the cap of daeth.
Replacing the lod electrolytics will be a doddle in this case as will replacing any cracked coupling capacitors.

[drewl]

I did a later one with power trans last year that was in dead mint condition, looked brand new

Owner had the matching guitar, also mint condition

[mikeydee77]

It goes without saying, fit an isolation transformer, 240 in and 110 out.
Isolation will make it safe after you remove the lower C4 the cap of daeth.
Replacing the lod electrolytics will be a doddle in this case as will replacing any cracked coupling capacitors.

Did you have one in mind Jon? I was just looking at Hammond ones which I rate as a brand and I was quite shocked by the price at circa £250.

Cheers
Mike

[Mick Bailey]

I came across an unlikely source of small isolation transformers a while back. They are intended for use in 240v industrial control panels where all the switches etc. need to be isolated for safety. Conveniently, the ones I bought were centre-tapped (120v-0v-120v). This gives a slightly higher output, but no worse than running an old amp off modern USA supplies. I've used these both for converting amps and also for building amps. The catch is they usually have a screw or quick terminal block on the top and need an enclosure. They start at about 50VA.

Similar to this type, but conveniently some of these have 110v outputs; https://tlauk.net/transformers-psus-...mers/scl500110

[mikeydee77]

I came across an unlikely source of small isolation transformers a while back. They are intended for use in 240v industrial control panels where all the switches etc. need to be isolated for safety. Conveniently, the ones I bought were centre-tapped (120v-0v-120v). This gives a slightly higher output, but no worse than running an old amp off modern USA supplies. I've used these both for converting amps and also for building amps. The catch is they usually have a screw or quick terminal block on the top and need an enclosure. They start at about 50VA.

Similar to this type, but conveniently some of these have 110v outputs; https://tlauk.net/transformers-psus-...mers/scl500110

Hi Mick,

This one looks correct for me https://tlauk.net/transformers-psus-...mers/scl300110 It's a good price.

[Jon Snell]

Did you have one in mind Jon? I was just looking at Hammond ones which I rate as a brand and I was quite shocked by the price at circa £250.

Cheers
Mike

https://uk.rs-online.com/web/p/toroi...ormers/3616377 should suffice.

It may even fit inside the amplifier cabinet ... bonus!

[Mick Bailey]

Hi Mick,

This one looks correct for me https://tlauk.net/transformers-psus-...mers/scl300110 It's a good price.

Check the size and where the transformer could be located. 300VA is higher than you perhaps need - did you work out the total load?

[mikeydee77]

Running the amp of a variac I measure the mains current as 600mA from the 240v outlet, with the amp on loud, and multiplied that by 240v to 144w and then was rounding up to 300 so yes plenty of safety margin. The other one they do is a 200VA rating. What would you advise ?

[Mick Bailey]

I would always choose a higher rated transformer to give better regulation and a better margin for overload conditions in the event of a fault. It comes down to space though. The control transformers are quite large compared with a toroidal.

In have in the back of my mind that the calculations for a Variac input/output current and voltage aren't as straightforward - could someone educate me on this? I'm going back to my college days and may be confusing this with something else.

[Helmholtz]

The OP multiplied the variac input current with the mains voltage.
That gives apparent input power, which includes reactive power and variac losses.
Assuming this was done for max. output, the 144W result is on the safe side.

Typically a tube amp PT needs to be rated at least at 3 times the audio output power (better 4 times).

[Mick Bailey]

This may be an alternative;https://cpc.farnell.com/multicomp/mc...55v/dp/FF01588