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 Post subject: Jukebox tube amp general questions
PostPosted: Mar Wed 04, 2020 2:17 pm 
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Hi all,

I have this 7868 pushpull tube amp in 100% original condition from 1971. It works great with no hum or distortion. Made in the USA.

It has carbon resistors, mostly disc caps, and the electrolytics of course.

My questions are:

- I don't want to replace the electrolytics because they are supposed to be of good quality and they are ok. Is it adviceable to keep them until any failure occurs? Very good quality caps of 450V are expensive, and replacing the original ones with cheap caps I guess will be worst. Here everything is really expensive and we have access only to very limited stuff.

- Should I replace the carbon resistors from the bias circuit and the plate loads and the ones in the power supply even if they are within expected values? From all the resistors I measured there are only three out of tolerance.... and the ones rated at 5% tolerance are amazingly sharp in value.

Of course the best will be to replace all with high quality components, but I want to go for the minimum intervention possible.
For example, I know these tubes are expensive so I am tempted to replace all the resistors associated to the bias circuit. But they are all in their correct values! Should I replace them with metal film ones just because?

I appreciate your comments and experience on this. Thanks.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Wed 04, 2020 3:40 pm 
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Joined: Sep Tue 15, 2015 1:16 am
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Location: 18424 PA
Good quality electrolytics are not expensive.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Wed 04, 2020 4:26 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
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Location: Smithfield, Pa. 15478
The electrolytics are open for discussion. If you can confirm all specs are correct. (Leakage, ESR, etc) you might be OK. But in any event be certain the power supply fuse is correct size and type. (If the first stage of filter shorts you want the fuse to blow before the transformer) But keep in mind that a filter cap further into the circuit, if it shorts likely will also take out a few dropping resistors. So with those conditions, you may be OK, but no promises. Do as you wish..

The resistors i would not worry about. Most carbon resistors, when they do fail, increase in value. Thus the bias would increase which would not cause damage. however it would affect the sound quality so they would need changed at that time. You could get away with letting those go till needed, but be aware of what to expect when or if they do fail.

I would be more concerned with the coupling caps especially those between the plates and control grids of the output tubes. Leakage here could cause excessive current and damage the outputs since this amp uses a fixed bias configuration. I would check / change those to a mylar type with at least 600 V operating spec. (I have found some of these in the ceramic tube to have excess leakage so beware!)


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Thu 05, 2020 1:50 am 
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Location: Rochester NY USA
Second the coupling caps - 7868 has high value grid resistors, so a little leakage will take out an output tube. Small electrolytics usually dry out first - the bigger cans hold more electrolyte. But all are suspect by now.

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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Thu 05, 2020 7:19 am 
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Thanks for you replies.

I will definitely replace the 4 coupling caps. I think I will let the original electrolytics on for now.

Two questions related to resistors:

- The bias voltage of the output tubes is negative. If the resistors related to it open the voltage will be less negative right? In this case -16.2V may become at most 0 if the resistors open completely. Won't it cause the tube to drain a lot of current as a possitive voltage from a leacky cap would do also?

- Metal film resistors are less noisy than carbon ones. Wouldn't it be advisable to replace the resistors in the power supply and the ones related to plates of the tubes to reduce noise?

Thanks again!


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Thu 05, 2020 4:48 pm 
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That is a Rowe amp. 25 watts per channel.

I do recommend replacing all carbon comp resistors with metal film.

Electrolytic caps may be ok IF the amp was pulled from a currently working jukebox, but if the amp has sat for a few years then you may want to replace them.

Concerning the 7868 tubes they are not real easy to come by (quite expensive for NOS) and the new manufacture 7868 tubes have base pins that are slightly too large and while they will work in an original 7868 socket, the pins will expand the contacts so that a NOS 7868 will no longer fit.

Suggest replacing the sockets with a good quality ceramic octal socket and using 7591 tubes as they are the same electrically as the 7868 which is what I did for that amp that is in the jukebox I restored for my aunt. seems to work great and most any new or NOS 7591 will work.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Thu 05, 2020 5:29 pm 
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You are not gonna find NOS 7591 for a decent price. There was a Russian sub for 7868? Do not know if it was pin compatable. I would look into 6v6/6l6 if the voltages and filaments would work.

Also, I would replace the few electrolytics before I would even think of replacing all the resistors with metal film. Why would you do that?


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Thu 05, 2020 7:01 pm 
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Not NOS sure, but new 7591 tubes are ok in this amp.

The one I repaired runs a quad of Groove Tubes 7591 and they work fine.

The key being this amp is only 25 WPC so the tubes aren't run near maximum.

For a matched quad of JJ 7591 it is $77.80

Can't find a NOS pair of 7868 for that low. Not even a new quad of 7868.

The 6L6 requires a higher drive voltage so it won't work.

6V6 won't work unless a quad per channel are used.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Thu 05, 2020 7:40 pm 
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JJ 7591s are known to put out lower wattage 15-17check yours. They are repinned 5881. EH get a better review. Would be better off to stay with 7868 or change to the magnoval socket if that bothers you.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Thu 05, 2020 8:13 pm 
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Oh ok did not know about those.

I used the Groove Tubes 7591.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 3:42 am 
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Groove tubes were Aspen Pittmans tubes he imported, and somewhat screened, later bought out by Fender. There were quality issues at certain points of ownership. They could be Russian, Chinese.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 5:36 am 
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Location: Olympia, Washington
That amplifier is likely out of a Rowe model MM5 or MM6 jukebox.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 12:20 pm 
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There is a new construction 7868 made by (or for) Electro-Harmonix. HOWEVER the pins are fatter than the vintage USA manufactured 7868 and they will ruin the tube socket the first time that they are inserted. They are about $35 each.

Here is an example of one listing:

https://www.thetubestore.com/electro-harmonix-7868

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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 1:11 pm 
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I'd replace the electrolytics before I'd even dream about replacing resistors that seem to be perfectly fine. Why replace good parts? If a resistor is on value, and not showing any mechanical "age", leave them be.

If it's working with those electrolytics in it, and you're not trying to market the amp as "restored", you could leave those for now as well, although if it were mine I'd probably replace them so I didn't have to come back later and do it.

That said, I've got plenty of test equipment way older than that, still working with the original electrolytic caps ...

if it ain't broke...... don't..............

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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 1:11 pm 
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Yes. It is from a MM5.
It has an output transformer with a lot of connections and implements mono bass which sounds absolutely deep and strong. You feel it in your body if the room has good accoustics.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 1:15 pm 
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3 resistors are quite cooked and for sure need to be replaced. But as they are part of the symmetrical output stage I will have to replace all the reciprocal ones too.
As far as replacing the good ones, carbon resistors are noisy and metal film ones are not. Thats why I was hesitating about replacing the ones in the power supply. And the ones in the bias circuit just thought about it because I wanted to avoid fryng the tubes if the resistors go open.


Last edited by normende on Mar Fri 06, 2020 1:31 pm, edited 1 time in total.

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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 1:29 pm 
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Location: Sunbury, Ohio 43074
[quote]Which ones are specifically worth replacing?/quote]

"worth" replacing is a concept that only you can answer. If your time is unlimited, and you want to replace every part in the amp, go for it. What is "Needed" is an entirely different question.

Look at it this way. Assuming that every carbon resistor is well within specs as to resistance value, what will be gained by replacing them with either metal film, carbon film, or plutonium film? If you are after an improvement of .0001% in "noise" , you may well realize that. Will you HEAR it? Doubtful.

Will the carbon resistors "age" and change in value in the future? Maybe, maybe not. They've so far withstood the test of time.

It's like making a decision to replace the piston rings in your engine after 100k miles even though compression still tests fine.

Hope that helps.

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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 1:53 pm 
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Ok thanks!


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 06, 2020 3:32 pm 
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I cannot remember exactly, but when I restored the MM5 for my aunt I'm thinking I did test some of the resistors which were found to be out of tolerance.

If the OP is repairing the amp then yes only replace the bad parts.

If the OP is restoring the amp then all parts known to eventually fail should be replaced.

Also keep in mind some new manufacture 7591 tubes (if you convert to those) may require a lower value of control grid resistor.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Sat 07, 2020 6:52 am 
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The questions are about what to do, but the first thing is to test the amplifier
before doing anything.

If it does work, there are benchmark things that can tell you if anything
needs special attention. Voltages for one should agree with what data you
have. Other tests may be outside of what you can do without appropriate
test gear.

The one thing that you have no control over is the condition of the transformers.
Another way of saying this, is that if you have a set of viable transformers ,
you can restore the amp to original condition.

Being a two channel amplifier, both sides must match.

If the amplifier is used to additionally feed speakers in a
location the juke is used, appropriate loads must be
used for the line voltage outputs.

I have no idea of the history, or the total working hours.

Like a taxi,only the owner/operator knows how old and tired it is.

50 or 60 years ago, those were on a tight leash. The same
people who furnished the records seemed to own and service
the boxes.

Is your juke going to collect money ?

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