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Old 26th Mar 2021, 3:25 pm   #1
RojDW48
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Default Modified Tripletone EL84 amp

I'm taking a look at this for a friend. 2 X EF86 + EL84 audio. Appears to have been modified to provide mic and line amplification - mic via a little RS step up TX. I am probably going to dispense with the mic facility. Does anyone recognise the model - so I can search for a circuit diagram?
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Old 26th Mar 2021, 3:42 pm   #2
GrimJosef
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Default Re: Modified Tripletone EL84 amp

Not sure about the model, but the TIY date code on that Hunts Mouldseal (nasty thing - check it for electrical leakage) is for week 43 in a year ending in 0, very probably 1960. So the amp will date from some time after that.

Cheers,

GJ
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Old 26th Mar 2021, 4:57 pm   #3
Edward Huggins
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Default Re: Modified Tripletone EL84 amp

Well that been well got at with the extra stage inserted between the first AF and the EL84. BTW, Tripletone never used EF86s, they used the 6BR8 alternative. It may be very hard to find a circuit for this. I only have its predecesor (Octal) valve circuit. But from servicing these way back, it's a very staight forward circuit, simple "cut" type tone controls (non Baxendall) and no NFB - but I cant vouch for the add-ons.....
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Old 26th Mar 2021, 7:38 pm   #4
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Default Re: Modified Tripletone EL84 amp

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Originally Posted by Edward Huggins View Post
Well that been well got at with the extra stage inserted between the first AF and the EL84. BTW, Tripletone never used EF86s, they used the 6BR8 alternative. It may be very hard to find a circuit for this. I only have its predecesor (Octal) valve circuit. But from servicing these way back, it's a very staight forward circuit, simple "cut" type tone controls (non Baxendall) and no NFB - but I cant vouch for the add-ons.....
Do you think that one of the EF86s has been plumbed in aftermarket Edward? I wondered myself as I have a little Tripletone with the same chassis - 6BR7 X 1 and the EL84. The extra valve base is very neatly fitted - more so than the additional wiring - ie, it looks original.
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Old 27th Mar 2021, 10:33 am   #5
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Default Re: Modified Tripletone EL84 amp

Tripletone never would have produced this variant. Why have a Microphone pre-amp with just 4 watts output - that would be no good for PA. It might have been modded as Guitar practice amplifier?
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Old 27th Mar 2021, 1:33 pm   #6
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Default Re: Modified Tripletone EL84 amp

I have attached two pictures of circuit I drew some time ago, as far as I can remember it is correct, but please check.
John
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Old 27th Mar 2021, 1:43 pm   #7
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Default Re: Modified Tripletone EL84 amp

145 ohms looks a little low for the EL84 cathode resistor. If you measure 8.9V across it then it's passing a touch more than 60mA (your HT rail says just 50mA). There's a touch more than 250V between the EL84's anode and cathode and much the same from the screen grid to cathode. More than 60mA times more than 250V is significantly more than 15W being dissipated inside the valve. Mullard say the maximum is 14W, and even that is running the valve very hard.

Cheers,

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Old 27th Mar 2021, 1:57 pm   #8
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Default Re: Modified Tripletone EL84 amp

Quote:
Originally Posted by Edward Huggins View Post
Tripletone never would have produced this variant. Why have a Microphone pre-amp with just 4 watts output - that would be no good for PA. It might have been modded as Guitar practice amplifier?

Plenty of oomph to drive a couple of outdoor PA horns!



Think of the equivalent of 40-50W into a line source (row of cone speakers in a box) PA stack.
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Old 27th Mar 2021, 2:14 pm   #9
Edward Huggins
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Default Re: Modified Tripletone EL84 amp

That circuit looks very familiar. You can see that when all 3 tone controls are set at maximum "cut", there is very little signal voltage going to the control grid of the EL84.
As stated earlier there no NFB loop, but the EL84 cathode is un-bypassed. Adding a 25uF 25V cap to it will give more volume, but distortion increases.
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Old 27th Mar 2021, 3:14 pm   #10
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Default Re: Modified Tripletone EL84 amp

Hi,

If your ok with an explosion in a spaghetti factory drawing style I drew out the circuit for itís bigger brother The Major a while ago, which can be found here https://www.vintage-radio.net/forum/...d.php?t=170726

Regards Terry
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Old 27th Mar 2021, 6:09 pm   #11
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Default Re: Modified Tripletone EL84 amp

GJ your observations are correct of course, I had a number of these amps on bench, with faults, so it looks like I made mistake in resistor value.
John
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Old 28th Mar 2021, 9:07 am   #12
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Default Re: Modified Tripletone EL84 amp

Quote:
Originally Posted by Edward Huggins View Post
Tripletone never used EF86s, they used the 6BR8 alternative.
You mean 6BR7. 6BR8 is a triode-pentode.
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Old 28th Mar 2021, 10:28 am   #13
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Default Re: Modified Tripletone EL84 amp

You're dead right. Sorry Folks! Now it's come up, I do have one of these rather rare valves acting as AF Amp/Phase Splitter in my Decca DERAM 205.
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Old 29th Mar 2021, 5:10 pm   #14
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Default Re: Modified Tripletone EL84 amp

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Originally Posted by John10b View Post
I have attached two pictures of circuit I drew some time ago, as far as I can remember it is correct, but please check.
John
Thanks John - I already have these diagrams - you kindly posted them when I was repairing my own little Tripletone - very helpful they were too !

I warmed it up gently and not for long this afternoon. The usual problems asserted themselves, so I shall replace the customary caps and clean the pots and see what transpires. It sounds perfectly viable though.
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Old 30th Mar 2021, 3:04 pm   #15
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Default Re: Modified Tripletone EL84 amp

These are great little gramophone amplifiers. They made quite a few versions but all basically similar.

This example 6BR7 EL84 and EZ80 sounds really good despite the smallish output transformer and lack of negative feedback.

Servio Radio, the original makers, were not far from my shop and I was always calling in for the odd component when I ran out. John.
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Old 30th Mar 2021, 11:15 pm   #16
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Default Re: Modified Tripletone EL84 amp

Quote:
Originally Posted by Heatercathodeshort View Post
These are great little gramophone amplifiers. They made quite a few versions but all basically similar.

This example 6BR7 EL84 and EZ80 sounds really good despite the smallish output transformer and lack of negative feedback.

Servio Radio, the original makers, were not far from my shop and I was always calling in for the odd component when I ran out. John.
That is a properly pretty refurb !! The same amp as mine (not the friend's in the post). Yes they are real sweeties - and your refurb is a joy to behold.
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Old 31st Mar 2021, 7:35 am   #17
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Default Re: Modified Tripletone EL84 amp

In terms of "refurb", these ampifiers were always so beautifully crafted. They all looked a real labour of love with the neatest hand wiring I've ever seen.
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Old 31st Mar 2021, 8:54 am   #18
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Default Re: Modified Tripletone EL84 amp

To be honest other than a few caps that I replaced it is exactly as built originally. It took less than an hour to complete and it has not been cleaned in any way.

It is used in conjunction with a 1930's Collaro 78 playing desk through three factory type loudspeakers to provide 'Music While you Work' in my collection shed! J.
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Old 31st Mar 2021, 12:22 pm   #19
Edward Huggins
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Default Re: Modified Tripletone EL84 amp

[QUOTE=RojDW48;1359034]
Quote:
Originally Posted by Heatercathodeshort View Post
These are great little gramophone amplifiers. They made quite a few versions but all basically similar.

This example 6BR7 EL84 and EZ80 sounds really good despite the smallish output transformer and lack of negative feedback.
I know they are rather small. A similar size was used on the earlier 6V6GT version. I think they were especially wound for Tripletone as I have never seen ones with connectors on the top like that anywhere else. Also, if they were especially wound, they would have specified a close match to the EL84 anode load. Note also the rather special pots, rarely seen elsewhere, with the rear connecting stub spindles for the add-on stereo adaptor chassis. As you've already guessed by now, I could go on and on.....
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Old 31st Mar 2021, 2:57 pm   #20
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Default Re: Modified Tripletone EL84 amp

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Originally Posted by Heatercathodeshort View Post
To be honest other than a few caps that I replaced it is exactly as built originally. It took less than an hour to complete and it has not been cleaned in any way.

It is used in conjunction with a 1930's Collaro 78 playing desk through three factory type loudspeakers to provide 'Music While you Work' in my collection shed! J.
I have their little stereo integrated amp from 1958 running the audio from this computer.
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