Grounding two mono tube amps like Eico HF22 for stereo

Dandy

Addicted Member
Please can someone tell me how to connect two mono tube amps to a preamp and avoid a ground loop? I'm trying to use two Mullard 5-20s for stereo with stereo tube preamp.

I followed these build directions carefully and each one is silent when operating in mono: http://www.r-type.org/articles/art-003d.htm The problem is when connected to one another there is a ground hum that I can't eliminate.

I think the Eico HF22 use the same circuit, so I'm sure this is a problem that that others have cracked.

The preamp works without hum on other stereo power amps, such as a Dynaco ST70.

Thanks for any help.
 
Strange, I don't have any ground loop noise with my20's, but I did pull v2 and run them into the tape out.
 
It can surely present problems sometimes.

First, determine what the actual ground loop is. If the inputs of the amps are connected together by way of a common ground at the preamp, then what is the other element to form the loop? Are the commons of the speakers connected together as well? If so, this together with the inputs connected together has now formed a loop. How about your AC plugs? Do you have ground prongs that are connecting the amp chassis together by way of the AC mains supply ground? Determining what the actual loop is will help determine the best approach for resolve.

One common trick to help eliminate such hum is to break the ground connection at the input jacks, and add 10 ohm resistors there. Such resistors are small enough in value to still effectively make the connection, but large enough to significantly reduce the ground currents to minimize the ground loop noise.

I hope this helps!

Dave
 
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The HF22s have two-wire power cords, so they really don't get into ground loop issues, normally.

Regards,
Gordon.
 
Agreed Gordon. I only mentioned that because the OP is in the UK, so I did not want to take that for a given, or not consider the possibility that the line cord sets may have been changed out somewhere along the way in the idea of safety.

Dave
 
Thanks bk and Dave. Bk, these amps are power amps similar to the HF22s, not integrated monos like the HF20s.

Dave thank you for focusing the questions of where the loop is. Both amps are grounded with IEC sockets. The ground of one goes to the busbar and the of the other to the chassis. Inputs of both are grounded to the busbar. These were my first build from scratch attempts and were built about a year apart. Once I understand what needs doing to fix the ground loop, I'll consider standardising ground arrangements for both.

When I have used them together just with a pot, I have removed the hum by lifting one of the pot grounds, but that 'pot in a box' is not with me at present. I tried disconnecting the grounds to the input of one amp, but this did not affect the hum. Neither was there an improvement by lifting the ground link from power inlet to chassis.

But, if I lift the ground to the preamp power socket the hum goes away.

I just wondered if there is a standard grounding layout to follow when two matching monoblocs are used in stereo? Gordon, would the HF22s not be use three wire power cords when rebuilt?
 
There really wasn't any such thing as a 3-wire power cord, in the era these were designed. That didn't start to come about, at least in the USA, until the mid to late 1960s. Most of these amps were mid-1950s designs... they were not designed with third-wire cords at all. To use those, requires a pretty extensive re-work of the amplifier. IMHO, it's not usually worth it, due the fact that straight power amps aren't something that people are touching on a regular basis... the preamp, yes, but not the power amps...

In gear with three-wire cords, I have had cases where both amp grounds had to be lifted- or, the preamp ground had to be lifted. Sometimes, you can't have both amp and preamp grounded. The only way you'll be able to do that, is to COMPLETELY separate the amplifier signal and power grounds from the chassis ground, and install a "ground lift" switch between the two... that way, you can have the chassis grounded to the third wire in the power cord, and the internals of the amp grounding through the input cables, by opening the ground lift switch...

Regards,
Gordon.
 
Thanks. So, lifting the ground of the preamp, and leaving both power amps grounded is not bad safety practice? At present the preamp power socket ground connection is grounded to the preamp chassis in one place only.

As this Cadence preamp was designed and built locally in small quantities, I attach some pics of it. It runs two 5814 tubes and there is no phono stage.
 

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OK. So the loop is formed between the ground of the amp/preamp interconnects, and the AC mains ground. This is rather typical.

The movement to improve safety of this old equipment is well intended, but often causes many problems like you are experiencing. If you suggest not using power cord grounds, the safety crowd gets their dander up.

Gordon covered the topic well for pre-code US equipment. In the UK, it might be different. But I have asked for anyone to find any record of someone being killed by this equipment for lack of an AC ground. But what if a fault occurs in the power transformer is the cry. I've got to go to the store now, and I'm rather certain that I have far more chance of being taking out in a common traffic accident than I do by the power transformers on my HF-22s.

Dave
 
lol @ Dave's comment, me, I'm picturing the look of Margaret Hamilton's gaze upward as Billy Burke tells her she better watch her step in Munchkin land before some one drops a house on her. lol
 
I like your perspective Dave :). I guess a lot of safety code provisions are designed to protect people who want to operate their toasters while bathing.

Bk, I have two mono integrateds waiting for attention (Avantics). I've not been able to sort out issues with their preamp sections, so plan to use them in the same way as your HF20s. So, thanks for the reminder. What preamp are you using, the ST84?
 
A follow up on this: my problem is solved by lifting the grounds links to the power socket on both power amps. Doing it this way, I can retain the ground on the newer preamp. There is no potential difference between the chassis and disconnected ground pin of the the power amps. If I leave it this way, are there any other safety checks I should make or measures I should take?
 
I abhore 3wire power cords. I always use "old fashioned" 2 wire cords on my "old fashioned" tube amps.
Thats what they came with and thats good enuf for me.
No "old fashioned" tube amp I own ever killed anyone.
 
A follow up on this: my problem is solved by lifting the grounds links to the power socket on both power amps. Doing it this way, I can retain the ground on the newer preamp. There is no potential difference between the chassis and disconnected ground pin of the the power amps. If I leave it this way, are there any other safety checks I should make or measures I should take?

Don't know the details of how it works on circuits there, but if you are worried about getting electrocuted then perhaps a GFCI is worth consideration. GFCI doesn't eliminate the possibility of receiving a shock, but it greatly reduces the likelyhood of being killed/electrocuted.
 
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