18 Watt Marshall clone general question about EL84 max plate current

[Planobilly]

Hi,

I have a four year old 18 watt Marshall 1974X reissue original amp I am working on for a friend. The cathode resistor that came in the amp is 91 ohms. That causes the plate current to be around 50ma. The tubes are not red plating but that is hot as hell.

I just finished a Mojo kit for someone and it ran at above 40 something ma with the original 125 ohm resistor. I changed the resistor to around 200 ohms and brought the plate current down to around 32 ma.

What is up with all this high plate current and what are you guys running your amps at?

Thanks,

Billy

 

[sluckey]

My 1974 runs 37mA with 150Ω. My Dual Lite runs 33mA with 150Ω. The EL84s are too hot to touch with a naked finger.

[kagliostro]

VOX ruled, they had 47R cathode resistor on some old AC30, this way the el34 were at ~110%

no chance for a long life of the tubes

but about tone .... you know

Franco

[PRR]

Plate/screen voltages matter too.

[Planobilly]

I assumed as much. Running the EL84 which has a max "normal" plate dissipation of 12 watts at more than 12 watts, is, I guess is a pretty common way to do things. My guys in the band are burning through tubes pretty often. I am not sure the change in tone and volume is worth the cost.

Everyone seems to like the tone of the clone I built which is running at 31ma...if I remember, the plate voltage was around 350 VDC.

Thanks for the info on what you guys are doing.

Billy

[kagliostro]

Swap el84 with 6P14P or better, 6P14P-EV tubes

http://www.tubes.ru/techinfo/HiFiAudio/6p14pev.html

Franco

[frus]

JJ's can take a lot. A guy was running two of them in a four-tube cathode-biased Laney for two years (at 16W/tube), no red plating, still sounding good

[Planobilly]

Hi guys,

Franco...I would like to try the 6P14P IF I could find a reliable way to buy them.

The issue I have with tubes is getting consistent stock.

For some time I acquired tubes from CE Distribution ans a couple of other smaller vendors. The results was very inconsistent. About a year ago I started dealing with Eurotubes. I have only acquired around 100  tubes from them so that is the extent of my experience with that company. All of the power tubes I have received from them have been matched within three ma. The one exception was a quad set of EL84s.  One of the tubes was 5 ma different. I spoke to Eddie and they sent me a replacement of the correct value at no charge plus told me to keep the hotter  tube. Great service.

I took a brief look on ebay in regard to the 6P14P. There are plenty for sale. I have no idea who to try to buy them from. All the advertisements are from Russia. I see lots of 100 for $700 plus shipping. I assume I could well matched tubes from a lot of 100 but I have no need for that many tubes and certainly have no desire to sell tubes.

I guess I could split the cathode to ground circuit from the current arrangement and use two resistors and two caps to deal with a set of mismatched  6P14P tubes.

Everything in electronics is a compromise at some level.

The issue with the 18 watt 1974X reissue that the band is using and bought new is reliability. Besides tubes not lasting very long, the output transformer has failed, the power transformer has failed, and this week the Celestion G12 M failed. I replaced the two transformers with Mercury Magnetics Radio Spares. How much better they are than the original Dagnall transformers, only time will tell. Dagnall power transformers in 18 watt Marshalls have a history of early failure. All this pretty much sucks for a amp that cost around $2400.

Cheers,

Billy   

[kagliostro]

This seems to be a good source for russian tubes (and now accept PayPal)

http://tubes-store.com/

but till now I didn't ordered there because I've find my tubes in one other way (usually at HamFest or on ebay from east europe vendors)

Ciao

Franco

[Planobilly]

Thanks Franco

Billy