Vintage amp mod, 12ax7(ecc83) plus 2 ecl82

[Lucaslz]

Hi,
I want to adapt an old philips 10 watt mono amp for guitar.
It has 2 inputs, 1 for turntable, 2 is for tape machine. I have plugged my guitar into both and channel 1 is very quiet and clean while channel 2 is louder and starts distorting at about half way. Turned all the way up it is very distorted which i like by the way but also very noisy.. However i am sure there is room for improvement so i decided to open it up. The architecture is a bit out of the ordinary and i wanted to ask what others thought or if they have seen something similiar, as well as any tips for improvement.
Basically channel 1 goes directly into a volume pot then thru a 300k resistor then thru a small brown tube with a wire wound at each end ( not sure if this is a resistor or inductor) then into pin 7 of the 12ax7.
Channel 2 has a 100 k rsistor going to ground then a 0.1/125v cap, then it goes to pin 2 of the 12ax7.
From pin 1 of the 12ax7 the signal then goes to a second volume pot but first passes thru another small brown unidentifuable component, also a 220k resistor and a 8uf/300v electrolytic cap going to ground.
From the second volume pot the signal then pases thru a 300k resistor and thru the same small brown tube with a wire at each end as channel 1 did, and then onto pin 7. pin 6 takes the signal to the tone controls, and then there are two ecl82's which are triodes and output pentodes in one glass.
Basically i want to mess around with the preamp circuit and try to optimize it for guitar. Hoping for a nice practice amp in the end. I have looked at alot of circuits and have never seen a volume pot before the amplifying stage. Also although channel 2 is amplified first then has volume, then a second stage of amplification, channel 1 and 2 do not interreact. I was thinking it might be nice to bridge the two channels to get a nice clean channel then an extra gain stage for crunch.
Any ideas on how to improve this circuit? And how to identify those brown tootsie rolls?
Thanks
Luxas

[Jazz P Bass]

Your best bet is to sit down & draw the circuit.
Scan it & post it.

[eschertron]

Your best bet is to sit down & draw the circuit.
Scan it & post it.

+1

A nice thing you could do when you chart out the circuit would be to give each component a number: R1, R2... C1, C2... so that we can talk about the components or connections between the components directly. Making a schematic also helps you recognize and understand the circuit as much as it helps the other posters.
Also, gut shots of the unidentified chocolate pieces might help.

[Brad1]

Does the amp have a model number on it? If so, have you searched for a schematic?

Do the brown tubes have any markings on them? Numbers, letters, stripes...anything? Maybe it will help determine what they might be if you measure with a meter? An inductor usually won't read real high resistance (it's just a coil of wire). On the other hand, if they are very low resistance wire-wound resistors...I guess that's possible.

Wondering if the phono input has any kind of RIAA eq? (Maybe not...my old Masco has a "phono" input. Input straight to a 500K pot and then straight to phase inverter.) How old do you figure the thing is? Any parts you can find numbers on to get any clues?

Just some ideas.

Brad1

[eschertron]

Wondering if the phono input has any kind of RIAA eq?

From the description given, I'd surmise that the phono input was intended to be hooked up to a separate phono preamp, which might have the RIAA filters on board.

[Enzo]

If it was from a low rent record player, it probably had a ceramic cartridge. THose have a very hot output. That might explain the phono input just being a pot.

The brown things are probably just caps.

[Mick Bailey]

Low rent stuff doesn't usually have RIAA equalisation due the the inherent characteristics of the ceramic cartridge (assuming it's fed into a high impedance load, which generally seems to be ignored commercially). I've built a number of amps using ECL82s and they've worked out really well. You don't mention any NFB - the tiny OPT usually associated with these amps often relies on a fair bit of feedback to help out with the bottom end and reduce distortion to an acceptable degree.

Some of these old amps (and tape recorders) are a good source of Mullard and Telefunken tubes. Probably hundreds of hours of use (or three hours for a tape recorder once the novelty wore off), but operating within the manufacturer's spec, so plenty of life left for guitar use.

[Lucaslz]

Philips EV7204 preamp

Hello and thanks for all the feedback.
The amp is a philips ev7204/00 mono amp. EV means it was made in Austria. There is nothing on the internet that I can find except a brief mentioning in radiomuseum.org. It was made around 1960. I haven't got any pictures of the guts yet but will make some tonight. The tootsie rolls are brown hollow cylinders with a thick wire wrapped on each end. There is some writing on them but I cannot make it out (it is a very tight fit in there and I have had to use little mirrors to read component parts). I measured them with a multimeter and got a reading of 1000k and 10,000k respectively.
About the sound; through channel 2 this amp has a really nice guitar sound. It is very expressive, nice harmonics, and good break up when turned half way up. Beyond that it starts to get out of control, and i start worrying about the speaker. I am sure Jimi would have liked it though!
Channel 1 is very tame and must be turned all the way up to hear anything, and remains clean.
The tone controls which follow the Ecc83 I have not drawn yet but are 2 pots, treble and bass, and are very responsive and interactive.
I tried bridging channel 1 and 2, and although this works it does not accomplish much. The channel 1 volume pot then becomes like a presence control for channel 2.
I have done my best at drawing up a schematic and present it to you for your consideration.
thanks again,
Lucas

[Enzo]

The junction of R3 and C2 should probably connect to B+.

Your mystery parts sure look like they need to be caps. Otherwise we winf up with plate voltage from pin 1 getting to the volume pot for channel 1 and also the grid at pin 7.


1000k and 10,000k are more commonly written as 1M and 10M or 1 meg and 10 meg.

[eschertron]

The junction of R3 and C2 should probably connect to B+.

Your mystery parts sure look like they need to be caps. Otherwise we winf up with plate voltage from pin 1 getting to the volume pot for channel 1 and also the grid at pin 7.


1000k and 10,000k are more commonly written as 1M and 10M or 1 meg and 10 meg.

Lucas,

Whenever I measure a cap with my ohmmeter, I get a reading that rises (as the cap is being charged) until it reaches a 'very high' number. If you measured them in-circuit, then all the components in the circuit, including any resistors on either side of the component under test can be part of the measurement.

As Enzo schooled me on ceramic phono cartridges (above), I'm sure he's right about those components being caps.

With a speaker that you can trust not to fail, it sounds like you've got a nice 2-channel amp to play with, and learn from!

edit: As a single triode is an inverting amplifier, when you jumper ch1 and ch2 together, the signals subtract instead of add. Maybe an A-B switch to select each channel would be an option?

[eschertron]

I just noticed both stages are grid-leak bias. Is that unusual? Does this limit the amount of gain stage two can produce?

Lucas,
Look up 'cathode bias' and 'grid-leak bias' if you're not already familiar with those terms. It's interesting to me to see any stage beyond the first stage have a grounded cathode.

[Enzo]

Probably both are grid leak because it was both cheaper and "good enough". As a guitar amp, I'd probably change it to cathode bias.

For a weak signal, the low grid leak bias is still enough, but is easily overdriven by a guitar signal. I suspect that is why channel 1 sounds OK but weak, while channel 2 is overdriven and sounds like it.

[Lucaslz]

pics

thanks again for your input,
I will have to revise the schematic as some new connections have been discovered. Basically The junction of R3 and C2 goes to a 220k resistor and then to pin 6. Also another wire from the same junction connects to a 27k resistor and then to a large multicap in this case the 32uf/350v side of it. As Enzo mentioned the B+ voltage is here and measures 250vdc. At pin 1 I measured 94vdc and at pin 6 I measured 91vdc. The chocolate cylinders have the value 10nf written on them so they must be a cap as all have been saying.
Here are some pics and as soon as I revise the schematic (I will also try to include the tone controls) I will post that as well.
Thanks
Lucas