18Watt.com

Hello all,

I've shifted this topic to by the bench - its going off topic for 18watters.
A quick query for you all.
I have a used mains transformer from a previous 18w project - a TMB I built last year. Unfortunately this PT puts out a bit more voltage than it is supposed to...
No load. 0-240 winding.....socket supply in 244v (resistance is 11.7ohms)
290-0-290 winding.....output 660v (resistance is 98ohms)
3.15-0-3.15 winding...output 7.63v
0-6.3v winding...output 7.65v
On load in TMB....
322v seen at rectifier plates with 396v at cathode.

It looks to me that the primary side could be overwound as all coils on secondary are showing high readings?
If this is so, then am I right in saying that to stay safely within the transformers rated power I am going to have to watch what current is being drawn? eg if previous secondary rating was 145mA then new max rating would probably be down to about 130Ma? I should say this is an old Ceriatone PT - rated i think at 145ma ht winding, 4amps 6.3v heater tap and 2a for rectifier tap.
I'm moving on to a new project now and i have been looking at some of the other schematics. I'm very interested in Brownnotes' D'lite amp using 6v6 tubes and 3 preamp tubes.

If i'm correct using heater current data as follows...
6L6 0.9amps each
6V6 0.45amps each
12ax7 0.3amps each
Total heater currents (with 6V6 power valves) = 1.8amps
(with 6L6 power valves) = 2.7amps total.
Either should be OK eh? I plan to run a S/S rectifier and a bias tap similar to markhuss's 6v6plexi.
does this all look reasonable? I'm still a bit of a newbie at this - coming away from a tried and tested layout is new territory for me.

comments appreciated - thanks

What was the heater voltage under load? Anything over 6.5 VAC is too much for good tube longevity. If it was OK under load, you should be alright with that PT as is with 6L6's, 6V6's, EL34's, KT66's. If your under load heater voltage was really 7.6VAC, there are at least two ways to use that PT.
1. Use the high voltage as is with the aforementioned output tubes but use a 5 watt 1 ohm resistor in series on one leg of the heater wiring.
2. Use a honkin big wattage resistor (say 500 ohms or so to start) in front of the PT to knock down both the HT and heater voltages at the same time. You could likely use EL84's in this instance.
Given a choice, I'd prefer option 1.

OK Thanks for reply.

On load heater voltage was definately over 6.5v, so I guess I'll just build next amp and measure volts after its complete - then knock it back with a resistor as suggested.
I think I'll go for D'lite with 6v6 valves to start with. Normster's has posted loads of info on his site. Has anyone else built this amp lately? - and how would it compare to an 18w tmb build in terms of volume, gain etc etc? Also has anyone out there built the D'lite with 6L6 tubes?

Thanks in advance

veets wrote:What was the heater voltage under load? Anything over 6.5 VAC is too much for good tube longevity.

Actually you only need to stay within +-10% of the nominal voltage for full tube life, so provided your heater voltage is below 7V you're fine. The important thing is to measure all voltages under load. If they're all still too high, you may need to use a Vintage Voltage Adapter, to reduce your incoming mains voltage - http://www.geofex.com/Article_Folders/v ... ntvolt.htm

Ok thanks for input - need to get some parts now - What about Barry (loverocker) anyone any news on his recent illness? - I had quite a few dealings with him over the phone and he is a good guy.

Hope he is doing OK.