EBS TD650 valve power supply

[MarkusBass]

I'm working on EBS TD650 bass amp. There is one ECC83 valve in the amp and it has very strange power supply. The heater is supplied from -62V through a 330 Ohms resistor. The voltage drop on the resistor is 51V so it means that the power dissipation of the resistor is 7.9W. Originally there was 7W resistor. I installed 10W resistor and measured its temperature after few minutes of work. And it's 140 degrees Celsius. According to the datasheet such resistor cannot handle 80% of load with this temperature. This is the worse design of the heater supply I have ever seen. What do you think? It's just asking for troubles .

Mark

[Steve Conner]

Let me guess... Marketing department wanted a tube, warehouse had a big stock of transformers with no 6.3V winding, design department had a hangover. :-p

[MarkusBass]

Let me guess... Marketing department wanted a tube, warehouse had a big stock of transformers with no 6.3V winding, design department had a hangover. :-p

Steve, this is not funny - this is most probably true. I had a slightly better opinion about EBS but it looks like I have to change it now. Any ideas how to solve the problem (except from buying a 20W resistor)?
There was one more problem with the amp: the rails are +/-62.8V and the capacitors were rated at 63V. And they didn't look good. I replaced them with caps rated at 80V. Again savings?

Mark

[J M Fahey]

Yes on both counts.
I'd rather buy a small 6.3V (or 12.6V) transformer ; wasting so much power and heat inside a chassis scratches me the wrong way.

[g1]

Sounds like your AC line voltage is too hot for the amp. Is it designed for use in Poland? Is your line voltage higher than the 230V it is supposed to be? JM Fahey's suggestion about the bucker transformer in series with the PT primary would be a good solution, unless there is a different tap on the PT which is closer to your line voltage.
There seem to be more modern amps now which can only handle maximum 115VAC (or 230V). They have no room for tolerance as you have seen with the filter caps.
There is a guy on here that believes they are designed for Japan (100V) and then labelled as 115V. But this is not the case. They are designed for 115V with no room for error. In the real world, north american line voltage can be up to 125/250V. Any of the amps with this problem behave normally if run on a variac set for 115V (not 100V ) Heater voltages come down to normal 6.3V, B+ levels on 63V filter caps come down to less than 63V. Power tubes biased too hot come down to acceptable levels.

[MarkusBass]

Thanks g-one. For a moment I thought that you are right and there is a problem with the transformer. But this is not the case. It's an european amp and the transformer has only one primary winding (230V as written on a sticker on the trafo). Measured line voltage is 234V - this is less than 2% above expected value. The secondaries should be (reading from the sticker) 2x42V. Without a signal I have 2x46V - slightly more but this is a 2 Ohms amp so with a load it will be 6-8 Volts less. On the capacitors I get 2x62V. It means that the amp is designed to work only in countries with 230V mains voltage. I think that for a moment in UK it was 240V (but maybe not now).
The voltage drop on the 330 Ohms resistor is 50.5V (power dissipation 7.7W) and there is 11.5V on the valve. I think it should be 12.6V so there is room to decrease the resistor slightly - maybe 300 Ohms will be OK. Anyway, I don't see any problem with the transformer or with incorrect connections to the main board. It's just designed in this way. I wonder how it survived for the last 10 years. The board around the resistor is brown from the heat.
The idea with an additional trafo is good but I would need also a bridge, capacitors and this may require more space. I will think about it.

Mark

[g1]

No additional bridge or caps are required. The additional transformer is placed in series with the PT primary, but out of phase. So it reduces the voltage to the PT primary by whatever value transformer used. For example, if you use a 6.3V transformer, your PT ends up getting 230 minus 6.3V, or 223.7VAC.
Here is the link to R.G.'s info. You will have to adapt it to 230V instead of 115V.
Vintage Voltage Adapter

[g1]

Sorry for the confusion, I just realized JM was not talking about a bucker transformer, he was talking about a separate transformer for the heaters.
The bucker transformer/vintage voltage adapter would also address the filter cap issues, but you have already solved that problem. Anyway, the vintage voltage adapter would reduce all the voltages in the amp, hope it makes sense.

[MarkusBass]

Yes, I understand what you ment. This is a clever solution but this is not what I need. I need to descrease the power dissipation of the resistor that is in the heater supply circuit. Decreasing the rails by 5-10% won't help much in this case. For the moment I put two 150 Ohms/10W resistors in series. I need to keep the modification within certain budget. For myself I would use a separate transformer as JMF says. I'd like to see the schematic of the amp - has anyone got it?

Mark