Stepping down voltage for power tube plate feed

[taddy]

Hi,

I have an old Seymour Duncan C60 Convertible and am thinking about stripping out the circuit in order to put in something more 'Fendery' - possibly a deluxe reverb. However I want to use the Seymour's existing transformers both the power trans and if possible the output trans (and possibly the solid state rectifier).

The prob is that the output voltages on the Seymour are a little high - around 512V as the C60 uses EL34s as I recall. Can I use resistors in order to step down the voltage to about 415V for the center tap on the OT? This would be more appropriate for 6V6 output tubes.  I know that a valve rectifier would bring it down as it is less efficient than a diode version but I'm not sure I want to start drilling large holes in the chassis in order to accommodate this and I'm not sure that the power trans has the correct heater/cathode feeds for a rectifying tube.

Thanks guys!

Tom

[Ed_Chambley]

Center tap might be best to use a zener diode. You can put a resistor right after your diodes, but probably add a little sag.


To use the output iron you will probably want to hook 8 ohm to a 16 speaker doubling the reflected impedance.


I wouldn't use the the power Trans, but you can do it. Just a lot of current to throw away and additional heat.

[92Volts]

If you want to go 6V6 it would be "ideal" to have a higher OPT impedance as well as lower voltage. JJ 6V6S tubes should take 500v though, as long as you run the screen at a lower voltage. Dropping screen voltage can be challenging but less overall power is wasted (therefore smaller components can be used) than dropping plate voltage.

With that said, it might be best to stick with EL34s. The preamp, phase inverter, and (to whatever extent you can change it) the design of the output stage may have a bigger effect on the sound than the tube type.

[PRR]

If it was a EL34 amp, stay with EL34 size tubes. This can include 6L6, which is as "Fendery" as you can get. Or 6550/KT88 (big 6L6) which will be comfortable at 60W for a long-long time.

Most of Fender's 6V6 preamps/drivers can be worked with EL34/6L6/etc finals. As 92 says, this may be a lot of the "sound".

No, resistors don't work at this power level because the power amp demand changes from soft to loud. If it idles at 100mA and you use a resistor for 100V drop, when it tries to roar at 200mA you have 200V drop and a puny 300V B+. Resistors are sometimes useful for <20W amps which tend to run more constant current.

[kagliostro]

Using a "trick" or more at the same time, may be you can find an usable situation with the wished voltage drop

(zener - VVR fixed at a certain output value - choke input) but 100v will not disappear, they go in dissipation = heat

as PRR say, I don't think it's worth the effort

Franco

[mresistor]

Couldn't he use a choke input on the ps to lower the voltage,  the choke might be on the large side and expensive?

[jjasilli]

Mr. Natural says:  Use the right transformer for the job.  Nevertheless you can use a power resistor or zener to drop up to 50V.  In addition you can use a bucking tranny to drop lots of HT voltage, so long as you keep heater voltage above 5V.

[taddy]

Thanks for all the help everyone! Looks like a resistor is a bad idea and a zener with a high voltage drop is going to kick out a lot of heat. Think I'll abandon this idea. Just trying to think of a way to cut my costs as fender parts in the UK cost considerably more than in the US.

[kagliostro]

I'm not able to find a schematic of your amp (I find only preamp section)

http://el34world.com/charts/Schematics/files/seymour_duncan/seymour_duncan_convertible_60_preamp.pdf

However I think the best solution will be to use PRR council, power tubes that can afford a high B+ also if used for a low power

less expensive than to get a new PT and, may be, OT

Franco

[taddy]

Thanks franco. I do have a pre and power amp schematic for the c60. It was all on the web but now been taken down. I think ill just bite the bullet and get a new chassis and transformers and build my own cab. I really want something a bit quieter than 60w!

[mresistor]

I've been down this rabbit hole before with a Baldwin Orgasonic PS chassis coverted to a H5F6A. But the Baldwin already had a huge choke that was utilized to configure a choke input PS. It dropped alot of volts. Mr. Natural was saying this is an option if you have a place to mount a choke on the chassis reasonably close to the PT and oriented 90 degrees from the PT orientation. There are hammond chokes that would work, but they aren't cheap. However one might find a used choke that is suitable. But as others have stated, it is probably more suitable for your cash situation to find the right PT and OT to begin with. Still I wouldn't give up on the endeavor. Originally when the Baldwin was in stock cathode bias configuration I had used 4 1N5353B zener diodes on the PT center tap and I think that dropped around 50+/-VAC .. I don't remember exactly, but they are rated for 16 volts. They are relatively cheap and worth a try. I mounted them on two three terminal standoffs in a space next to the PT. You might be able to use 5881's or you could use 6V6 like you are talking about. 450-460 volts isn't really a big deal.