Valve amplifier issues... Help Please

[Fretking18]

Valve amp questions

Hi Guys
My name is Stuart and I love tinkering with Musical instruments and old Hi-fi, though i have no formal training, i have more than a few years experience of fixing and fettling, and looking on your forum it looks the ideal place to discuss all thing Vintage Hi-fi.

However... I am new to messing around with valve amplifiers, so am looking for some advice from you old hands out there please.

I have recently purchased a 1962 Dynatron Mazurka Model GR7ST. It comes with a Garrard 4HF 4speed record deck with a TPA12 tonearm, and period correct Sonotone ST/8TA cart and stylus. And a Goodman full range Speaker and a Goodman Tweeter, both with Alnico magnets

The really interesting bit for me is the Power amplifier and Pre amplifier are both valve, powered with Mullard valves... To my eyes the Power amplifier looks to be stereo

It has a mono connector on the top for the internal speakers, and on the back at both ends what appear to be connectors for external speakers, but i don't know what type of connectors to use to try out a pair of speakers as standard banana speaker plugs don't fit they are too big.

What i need to know please is:

1) What special precautions do i have to take in regards to handling the unit in regards to Electric shocks. I'm led to understand they can be really dangerous if not handled correctly. How do i avoid being electrocuted

2) Does anyone have any idea which company made the valve amplifiers for Dynatron Mazurka during 1962

3) The pre amp can only select from Phono or radio, yet on the back of the power amplifier are two extra inputs one which looks like a single mono RCA input and the other is a female Din plug, which a previous owner have kindly marked up as Cassette deck two. (see photo)... so how would i select these extra inputs ?

4) Wired directly to the Power amp on/off Switch there are two cables coming out the back of the amplifier which end a some kind of cable joiners. Could these be auxiliary power sources ? They are the same width and look as standard hi fi power leads.

5) On the top of the power amp are two adjusters for two internal potentiometers wired in to the circuit board. There is no mention at all in the owners manual what these are for. Generally speaking the user would not be aware of their existence unless like me they removed the Power amp out of the Chassis.

6) In regards to the speaker connections is there some kind of special 1960's valve amp speaker connections that I'm not aware off or could they be the smaller 2mm banana plug connectors.

I can upload further photographs if required

Thank you in advance for any advice or Help you may be able to give me to identify and safely use this valve amplifier

[Records]

From what i can see the connection on the right hand side of the pic is for an extension speaker to replace the one that comes with the unit, it turns off the internal speaker.
The left hand speaker plug gives you an output to plug into another amp to drive another speaker, that could possibly be stereo, it depends how the cartridge is wired, i very much doubt the original valve amp is stereo, i maybe wrong.
The amp chassis has a mains transformer so it should be earthed, but as with anything mains dont go fiddling around with it plugged in unless you know what you are doing.

[TonyDuell]

From the photos it looks like it was originally a mono valve amplifier that has been 'modified' to stereo by adding a second channel -- using transistors (!).

The original amplifier is the right hand part in the photo. The 3 tall valves are probably for push-pull output and the HT (high voltage for the other valves) rectifier. There are 2 transformers in that part, the larger one is probably the mains transfomer, the smaller one the output transformer to link the output valves to the speaker.

The shinier piece of aluminium on the left is the modification. It has another transformer (probably mains supply for the second channel) and what looks like a Mullard LP something-or-other audio amplifier module with a couple of power transistors on the heatsink.

I am not sure what sort of stereo you will get from 2 totally different amplifiers like that.

And given that it has been modified, I think the only way to know how to use the extra inputs, etc is to start tracing out the circuit.

[HamishBoxer]

To get back to the original question,use an isolation transformer and work with one hand in your pocket.

Failing that have it looked at by some one experienced.

[Edward Huggins]

To try to answer a few more of your queries, Dynatron in Maidenhead made the amplifiers. They did not outsource. You have a really good deck and arm in your version. Some of these chassis were stereo adaptable and also have provision for a seperate FM only tuner. I don't think you have the connections for these. I may be missing the obvious, but that sub-assemply on the LH side of the chassis does not look original and it appears to have it's own mains transformer - it might have replaced the otiginal mains transformer should this have failed?.

[ms660]

LP1173 module?

Original extra amplifier?:

http://www.vintage-radio.net/forum/s...d.php?t=122088

Lawrence.

[Edward Huggins]

The above link is relavant and looks just as I would expect, just like the other channel being powered from the mains PSU. Even so, I still can't work out what that (less aged) subplate chassis bearing another large TX is for?

[Fretking18]

Originally Posted by Records

From what i can see the connection on the right hand side of the pic is for an extension speaker to replace the one that comes with the unit, it turns off the internal speaker.
The left hand speaker plug gives you an output to plug into another amp to drive another speaker, that could possibly be stereo, it depends how the cartridge is wired, i very much doubt the original valve amp is stereo, i maybe wrong.

Hi thanks for your reply..I read a thread on the Vinyl engine website which described the difference between how the stereo and the mono cartridge should be wired up in this particular headshell and mine is wired in the described stereo configuration i will up load a photograph so you can see what you think

[Fretking18]

Originally Posted by TonyDuell

From the photos it looks like it was originally a mono valve amplifier that has been 'modified' to stereo by adding a second channel -- using transistors (!).

Quad use two mono block power amplifiers however the difference is that the speaker connections are on the pre amp no the power amp.

I have no idea when it was modified or by whom..it above my knowledge level

[superdirto]

Hi
They were/are/mine is stereo *(1963) afaik one of the very first stereo record players as the secong speaker is housed in a duplicate but dummy player case complete with legs somewhere I have the original bill of sale etc and warranty card lol pretty much a mullard 10 watt type design very high quality and with a 9t sonotone high compliance cart (TA?or AT) was a crystal cartridge but very few survive very nice sounding in allwith the pre amp valve by the volume knob module ( makes sense )
yours looks to be missing one channels output valves on the left and there are no transistors in the original

[Fretking18]

I've added an extra photo of the cartridge wired for stereo and the original owners manual lists the model no GR7ST as being wired for Stereo my unit model is the GR7ST..is it possible that the Mono power amplifier was made Stereo by the manufacturer in 1962..dont forget at that time most people would not require a stereo Power amplifier as most records were still Mono back then..so a stereo amplifier would be a specialist item which Dynatron might build off their existing technology platform

I have also uploaded the underside of the modified section

[Fretking18]

Originally Posted by TonyDuell

I am not sure what sort of stereo you will get from 2 totally different amplifiers like that.

HI Tony thanks for your reply. There is only one way to find out I will have to connect a set of speakers to it and see what happens and find out if it is in stereo

[paulsherwin]

This isn't a good project for a beginner, as it has obviously been seriously Got At. The right side of the chassis looks like a conventional push-pull valve amplifier, presumably original. The left side consists of a transistor amp from the late 60s or early 70s, complete with its own mains transformer. You will need to decide if you want to repair it in its highly modified current state, or remove the transistor amp and return the chassis to its 1962 configuration. Neither of these options are easy for someone without appropriate experience.

[Fretking18]

HI Paul.. i don't have an intention of re modifiying it.. all that i want to do is get it working..which im pretty sure it does anyway

..oh by the way do you make guitar pedals ..im pretty sure i bought a Fuzz pedal off you about 7 or 8 years ago

[ms660]

Can you verify the model number on the rear cover?

Looks like extra sockets with labels have been fitted.

Lawrence.

[Fretking18]

Originally Posted by Edward Huggins

To try to answer a few more of your queries, Dynatron in Maidenhead made the amplifiers. They did not outsource. You have a really good deck and arm in your version.

Thank you Edward. I'm glad to know that Dynatron made the valve amplifiers themselves Thanks. I am aware that the version I have with the Gerrard 4HF is quite rare and sought after. The deck needs a little work it has the usual fault that the speed changer is stuck on one speed but I know how to fix that (hopefully).

[Fretking18]

Originally Posted by ms660

Can you verify the model number on the rear cover?

Looks like extra sockets with labels have been fitted.

Lawrence.

HI Lawrence
it say on the back cover TYPE GR 7SA made by Dynatron

It says in the Dynatron manual that the GR 7SA is mono but can easily be converted to stereo by your authorised Dynatron dealer by the addition of a second amplifier and that your Dynatron dealer will be happy to discuss the conversion to you, which only goes to make me think that this has been converted by Dynatron themselves at a slightly later point.

[ms660]

It looks like a DIY conversion to me.

Lawrence.

[paulsherwin]

I don't make guitar pedals, must be somebody else.

[Fretking18]

Originally Posted by HamishBoxer

To get back to the original question,use an isolation transformer and work with one hand in your pocket.

Failing that have it looked at by some one experienced.

Thank you Hamish. I don't have an isolation transformer and I can't afford to get it looked at by a valve specialist as that kind of expertise doesn't come cheap. I will just have to make sure I'm well earthed at all times. However I have no plans for modifications at all I will simply give it a bit of a clean up and put it back in the carriage once I've tested that it all works as it should.