Rickenbacker TR-25 Guitar amp - new Transformer needed

[abar121]

Hi Chaps, 

I have an old TR25 solid state amp that I was given, not currently working.    I believe it needs a new transformer at the very least, so starting from there.

Can anyone help, as my electronics knowledge is pretty patchy and out of date!  

I was thinking of using a 100VA or 160VA  240v 18v toroidal transformer, which has two 18v outputs on the secondary, so I would run them in series to get the 36v AC the rectifier needs.  Or would I need a 40v output given the rectifier losses?  Rickenbacker tech support have not been helpful.

There is a circuit diagram here, if anyone could spare the time and sanity check this for me.

http://www.rickenbacker.com/pdfs/19341.pdf

Thanks, 

Alan.

[Mark Phillips]

Hi Alan,

Firstly, if your knowledge is patchy then in the interests of staying alive it's best to think more than twice about repairing things yourself, especially where mains wiring (e.g. to a transformer) is involved. It's probably best to take it to someone with adequate knowledge. Having said that, and also that no responsibility is accepted for anything you do as a result of my input, here are a few general comments for you (I don't have any experience with this particular amp).

You say you believe it needs a new transformer, but if so then that should be very clear - e.g. by visible damage (burning etc), by making resistance measurements of the primary & secondary windings (with no power applied) and/or by making voltage measurements at the input and output of the transformer - possibly after disconnecting the secondaries from the rest of the amp. Transformers are expensive and fail quite rarely - especially if there is no other major fault in the amp that has caused it to fail - so you wouldn't want to be replacing it unnecessarily.

The schematic is very helpful - but it does show the US version, i.e. 120V mains. If you have a version with a 120V transformer fitted and it has been connected to UK mains (now nominally 230V - but is often between 240 and 250 in practice!) then it's likely that the amp has been seriously damaged - not just the transformer.

For a solid state amp, the VA rating of the transformer usually needs to be no more than about 1.7 times the rated audio power output of the amp. So, for example, if this is a 25W amp (I don't know) then a 40 or 50VA transformer would probably be fine, though you could go to a 60 or 75 if it was important to you to get the maximum power out of the amp. Scale these figures up, depending on the actual audio power rating of this amp. Another way to judge the transformer rating is if there is any info provided on the rating plate, usually on the back of the amp. Sometimes there is a mains input power, VA, or current rating given there. The mains fuse rating indicated (for a specific supply voltage) gives only a very rough idea, but the primary current rating of the transformer (which is roughly its VA rating divided by primary voltage) should be at least as large as the fuse value [Edit: not really so - see my later post]. The schematic seems to say both 1A and 3A next to the mains fuse, but the resolution isn't adequate to be sure what it really says. [Edit: again, see my later post.]

No, the secondary voltage does not need to be more than the DC rail voltage because of rectifier losses, as you supposed. In fact, you get an apparent 'gain' in voltage from the rectification process, because the reservoir capacitors charge up to (nearly) the peak voltage of the secondary's AC waveform, not its RMS voltage (which is the one that's quoted in the transformer spec). So, for 36V DC supplies, you need approx 25V secondaries (24V would be fine). As you say, two of those secondaries would be connected in series (observing correct polarity), with the junction between them connecting to what currently goes to the centre-tap of the transformer shown on the schematic (i.e. the terminal labelled something like "G2", which connects to earth/chassis).

Finally, note that, when replacing a transformer with a completely different type to the original, there can sometimes be issues with magnetically-induced hum. This is more likely when the new transformer is mounted with a different physical orientation to the original (e.g. replacing a conventional one with a toroidal). If the original transformer has magnetic screening or has a copper flux band around it, this can be an indication that the amp manufacturer found that a transformer with low flux leakage was needed to avoid hum issues.

I hope this helps you. Please take care.

Mark.

Edited May 30, 2019 by Mark Phillips
Added a note to refer to my later corrective post.

[Mark Phillips]

In the interests of accuracy, I just wanted to add a bit more to clarify/correct my comment on using the specified mains input fuse value as a very rough guide to the transformer VA rating. The selection of mains input fuse rating made by the manufacturer is (or should be) quite an involved process - not so simplistic as I perhaps implied. This is mostly because of the inrush current that flows very briefly when the amp is switched on (except in amps with a soft-start feature); this inrush current is much higher than the steady-state mains current, but must not cause the fuse to blow. Hence the common use of anti-surge ('T'-type) fuses, and/or use of higher fuse ratings than expected. (The actual value of inrush current depends mostly on the value of the reservoir capacitors used and on the design of the transformer, but isn't at all easy to calculate.)

Note that toroidal transformers tend to have somewhat higher inrush currents than traditional types.

Some other, less significant, reasons that the fuse may be a higher value than would be expected from transformer VA divided by mains voltage are:

• The VA rating is usually referenced to the transformer's (total) output capability, i.e. the secondary side. So the primary side takes a little more current than expected from VA, because of transformer losses - and the primary winding is of course rated accordingly. For example, assuming a transformer efficiency of 80% at full load, this would increase the primary side fuse requirement by 25% above what would be expected from the VA figure.
• Temperature derating of the fuse may mean a higher value of fuse needs to be used in order to cater for operation when the fuse runs at an elevated ambient temperature - this effect is more significant with power-inefficient valve amps, where the chassis can get really hot after prolonged use in a warm environment.
• Fuses come in standard ratings, and the next highest rating above the 'ideal' value has to be selected.

So all of this means that my implied suggestion of estimating your existing transformer VA rating by multiplying the fuse rating by the mains voltage isn't even roughly correct (mostly because of the inrush considerations). Except for amps with a soft-start feature (enabling close-rated fusing), that method would suggest a much higher VA rating than what is correct.

I've also had a closer look at the schematic, and I now think it indicates the fuse (for 120V supply) as 1A rating; I think the "3A" is part of the type designation 3AG, which is a US code for a 1¹/₄" glass-bodied type. As a very rough guess, taking all the above into account, I would suggest that a 1A fuse might be consistent with an original 120V transformer having VA rating in the range 40 to 60VA - as originally expected if this is a 25W amp. For the 230/240V version of the amp, a 500mA fuse ought to be specified.

[abar121]

Hi Mark, 

Many thanks for the reply and apologies for the very late response.

I did purchase a 220v 80VA transformer which was fitted.  I did some further troubleshooting as the output was very low / noisy.  I replaced one of the transistors that tested faulty, but that's as far as I got. 

It really needs someone with more expertise and a 'scope to diagnose further.  If anyone around the London or Bucks / Beds area fancies looking at a project,  that would be fantastic.

Cheers.