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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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26th Mar 2021, 4:25 pm | #1 |
Nonode
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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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26th Mar 2021, 4:42 pm | #2 |
Dekatron
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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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26th Mar 2021, 5:57 pm | #3 |
Dekatron
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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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Edward. |
26th Mar 2021, 8:38 pm | #4 | |
Nonode
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Re: Modified Tripletone EL84 amp
Quote:
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27th Mar 2021, 11:33 am | #5 |
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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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27th Mar 2021, 2:33 pm | #6 |
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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 |
27th Mar 2021, 2:43 pm | #7 |
Dekatron
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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, GJ
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27th Mar 2021, 2:57 pm | #8 | |
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Re: Modified Tripletone EL84 amp
Quote:
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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27th Mar 2021, 3:14 pm | #9 |
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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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Edward. |
27th Mar 2021, 4:14 pm | #10 |
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Re: Modified Tripletone EL84 amp
Hi,
If your ok with an explosion in a spaghetti factory drawing style I drew out the circuit for it’s bigger brother The Major a while ago, which can be found here https://www.vintage-radio.net/forum/...d.php?t=170726 Regards Terry |
27th Mar 2021, 7:09 pm | #11 |
Nonode
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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 |
28th Mar 2021, 10:07 am | #12 |
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Re: Modified Tripletone EL84 amp
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28th Mar 2021, 11:28 am | #13 |
Dekatron
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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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Edward. |
29th Mar 2021, 6:10 pm | #14 | |
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Re: Modified Tripletone EL84 amp
Quote:
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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30th Mar 2021, 4:04 pm | #15 |
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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. |
31st Mar 2021, 12:15 am | #16 | |
Nonode
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Re: Modified Tripletone EL84 amp
Quote:
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31st Mar 2021, 8:35 am | #17 |
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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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Edward. |
31st Mar 2021, 9:54 am | #18 |
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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. |
31st Mar 2021, 1:22 pm | #19 |
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Re: Modified Tripletone EL84 amp
[QUOTE=RojDW48;1359034]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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31st Mar 2021, 3:57 pm | #20 | |
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Re: Modified Tripletone EL84 amp
Quote:
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