ill amp 'crackles' into speaker ?

[FatGaz]

Hi,

My traynor reverb-master amp YRM-1 (early 70's) has started to show signs of being poorly, so I'll need to get the dmv and soldering iron out.

advice about symptoms liste below greatfully received !

available volume fluctuates -
sometimes it sounds clean, sometimes sounds a bit crunchy.

ALSO it has started to produce a crackling -scratchy noise from the speaker that is not affected by the master volume - ie the crackle is the same loudness - I can add normal white noise by turning the volume to max.

SO where should I be investgating first ??

power tubes ?? (i don't have any spares to hand at the moment)

I did swap the first pre 12ax7 and it made no difference.

coupling caps ?

power supply caps ?

thanks all !! and happy new year !

Gareth.


PS. it's running 6v6gt tubes at the moment (was el34) and I've moved the master vol post the PI - mods done many years ago and are ok... I may swap in a pair of el34's again - might not change the plate voltage though, it used to be quite high, but i dropped it down for the 6v6 tubes. will need a rebias of course (fixed).

[pecorporation]

I'd suspect the output tubes first. While the amp is running tap on the output tubes with a pencil to see if you hear a rumble noise through the speaker. If you do, the tubes are bad.

Other than that, if the volume does not affect the static and volume fluctuates, I'd look at the quality of the solder on all tube sockets. One trick is to use a drum stick (or wood dowl rid) to gently move wire & tap components (filter caps, especially) on the inside of the amp to help narrow down flaws. But be careful, high voltages can kill.

[FatGaz]

Other than that, if the volume does not affect the static and volume fluctuates, I'd look at the quality of the solder on all tube sockets. One trick is to use a drum stick (or wood dowl rid) to gently move wire & tap components (filter caps, especially) on the inside of the amp to help narrow down flaws. But be careful, high voltages can kill.

I wonder if the fact that the master vol is after the PI (and nosies are unaffected by this) narrows down the likely problem to the power tubes and their sockets / PSU caps / and screen resistor. ?

no harm in ordering some new tubes and maybe some new PSU caps though...

cheers,

Gareth.

(ps i tend to use plastic chop sticks for such amp pokery)

[Gtr_tech]

Yes, graphite is conductive......

[Jazz P Bass]

But be careful, high voltages can kill.

So can low voltages.

[FatGaz]

other little tasks ?

Hi,

I am picking up my new power tubes this evening - so will start doing some digging around in the amp.

any recommendations of serviceing I should consider ?
obviously, I'll check stuff looks ok.

or should I just get the tube biased properly and hope that removes the symptoms.

Gareth.

[FatGaz]

one step forward one step back - new tube dodgy ?

HELP ! 6ca7eh biasing problems. all advice gratefully received !

amp schem here : http://www.lynx.bc.ca/~jc/traynor_YRM1.gif

I have now got 430V on the plates as per the schem.

I have changed the bias circuit so that I have an adjustment around the original -36V - i'v put a 47K pot in series with R43 and a 47K in parallel with R41 (the original voltage divider was r43=47K and r41=39 K )

I set the (shared) bias voltage to -43V (at point C, schem says -36V) this ends up as -39V at the tube socket.

I measured the currents via cathode resistors as :

39mA and only 22mA - very low on one tube !!!!!

I have just swapped tubes - the low bias follows the tubes*

the voltages measure ok other wise :

good? tube (V6) plate - 432V, screen 425V , PI plate 205 V (V4b)

bad? tube (V7) plate =434V, screen 428 V, PI plate 229V (v4a)

HELP !

is one of my new tubes bad ?

or might something else be causing the problem.

Gareth.

* i think - not 100% sure if i remembered the 1st position correctly ! will double check in the morning (11pm here)

NOTE at 435V 60%-70% bias is 35mA to 40mA for EL34's I think.

[Jazz P Bass]

Bias

I would think that you have two different tubes.
I can't say whether either of them is "bad".
Certainly different.
How does it sound?
If push came to shove I would put the weak one on the upper part of the PI.

Groove Tubes would market these two as (39ma) +9 (22ma) +2
Please do not hold me to the numbers.
Tonque in cheek slap at GT, that's all.

[FatGaz]

how matched should matched tubes be ??

I would think that you have two different tubes.
I can't say whether either of them id "bad".
Certainly different.

Well, since the tubes are supposedly 'computer matched' I was hoping that the idle current would be closer than I have found it to be - say a 10-20% difference, not a 100% difference !!!

how matched should i expect modern tubes to be ????

They are 6ca7-EH sourced from a (supposedly reputable) mail-order co. here in the UK.

I believe I have the legal right to reject them, but I'm not sure if this is a sensible course of action.

any advice ?

Gareth.


PS - I did double check the tubes this morning and the low current does 'follow the tube' between socket positions. So I summise it's the tube not the circuitry....

[Steve Conner]

Definitely call the supplier and ask if you can send them back. The amp will surely work fine, but you paid for matched tubes.

I've only ever bought from Watford Valves and Hotrox, and both of them write the plate current reading from their matching machine either on the carton or on the tube base.

If those two numbers are close (mine were all within 2mA) then the tubes were matched when they left the supplier. The mismatch may be due to a pasting in the post, causing the innards to shift mechanically, but they might well replace them for you anyway.

[FatGaz]

Definitely call the supplier and ask if you can send them back. The amp will surely work fine, but you paid for matched tubes.

I've only ever bought from Watford Valves and Hotrox, and both of them write the plate current reading from their matching machine either on the carton or on the tube base.

Hi Steve,

It was from one of those suppliers, both tube boxes had '66' marked on them. I have only used the other supplier before.

If those two numbers are close (mine were all within 2mA) then the tubes were matched when they left the supplier. The mismatch may be due to a pasting in the post, causing the innards to shift mechanically, but they might well replace them for you anyway.

The supplier I used doesn't seem to have much in the way of T&C on their web site, I'll give them a bell and see what's what. I'm a bit concerned that the other pair of power tubes I ordered might be badly matched or damaged too. I'll have to see if my other (diy project) amp is running sufficently for me to test them.

I'll also have a close look at the tubes to see if there is any sign of damage in transit.

cheers for the advice !

Gareth.

(hope the weather isn't too bad up there in Glasgow, getting to work was fun this morning in the car... I have a beautiful winter vista from my office window today - really makes me itch to go skiing or mountaineering, rather than be sat at a computer all day...)