Ceriatone 36W mods?

[George61]

Mark,

I must apologize about the half power thing. That's my fault. I should have stated that I was going to remove the circuitry that doesn't work or that I don't need. The half power does work but since I am chasing clean headroom with this amp I will never use it.

The 90R thing I got off of 18watt.com, sort of. The 2200uF or 3300uF cathode capacitor appears to be a common mod to the 18 watters to tighten up the bass and several posters suggest changing the cathode resistor to 180R (for two tubes) also. If I am using one resistor for four tubes wouldn't I use half that value?

I have a really good article on fixed and cathode bias, which is a little over my head, but I will read and re-read it until some of it sinks in. If I am going to build guitar amps I really have to learn more about the circuitry and what the individual components do.

[Phil_S]

The typical wiring for the power tube cathodes is to join the inner pair separately from the outer pair. In other words, don't wire all four cathodes together in parallel. If you did that, then, yes, you'd halve the value of the cathode resistor. Ordinarily each pair gets it's own R/C, so using two 180 ohm resistors is the equivalent of 90 ohms.

The typical cathode resistor for a pair of EL84's is 130 ohms. People go up or down depending on actual conditions and the tubes. 125, 130, 150, and 180 are common values that all work. So 90 ohms isn't out of the question, but only in theory.

In practice building the amp and separating the pairs as I described, allows you to pull one pair, and your half power switch probably lifts the connection to one pair. If you have one common Rk, then your half power switch probably doesn't operate correctly. Using two R's also maintains bias on one pair if for any reason the other pair is heading south in a hurry. That alone is good reason to separate the pairs.

There is some math you can do to figure out if you've selected the correct cathode resistor. We will need tube socket readings for the quad to see what's up with it and we need to know for sure what Rk value you are using, as that's part of the calculation. I'll be happy to show it to you if you provide the data.

[George61]

I finally got around to playing with this amp. The idea of a cathode bias resistor and capacitor for each pair of tubes makes sense. I was going to use a 150R 10W resistor and a 100uF 50V capacitor for each pair.

Can I use a SPST toggle switch to disconnect the resistor and capacitor for one pair of tubes for half-power?

I think I know why this amp didn't work very good. One of the EL84 screen grid resistors was not soldered. It was hooked around the tube socket terminal but not soldered. Whoops!

Speaking of screen grid resistors, the kit came with 100R 3W resistors. From what I understand that is awfully low. I was going to use 470R. OK?

Thanks, in advance, to anyone who replies.

Solder on,
George

[alvarezh]

Hi George, here is the Ceriatone Liverpool Layout with the half power switch.

In order to calculate your bias resistor values you need voltage information which Ceriatone does not include on the layout. Contact Nik at Ceriatone directly through email, he can calculate the bias resistor values for the 36W EF86.

All the best.

[Phil_S]

Speaking of screen grid resistors, the kit came with 100R 3W resistors. From what I understand that is awfully low. I was going to use 470R. OK?
George

The schematic from 18watt.com shows 100R. I'd just stick with that and see how it goes. You can even leave them out and the amp will work, but I wouldn't do that. OTOH, if you want to use 470's, I don't imagine that will be any sort of problem.

[M Fowler]

I used 100R screen resistors on a reverb Rocket build.

This amp has the dual HT switch and 18w/36w switch.

I used 47R and 68R bias resistors.

Mark

[George61]

So I've got the amp re-wired. It's better than it was, not great or anything, but better. I eliminated the half-power switch and used a 150R 10W resistor and 100uF 50V electrolytic capacitor for cathode bias for each pair of output tubes. That's the currrent Richie Hall design on 18watt.com for his 36 watter.

What is a safe way to power it up for the first time? I have recently bought a variac.

Thanks in advanced for any help.

George

[George61]

I got the amp up and running and it sounds better but not great.

I checked the voltages and I seem to be about 19 volts high across the board from what the Ceriatone voltage chart lists. Here's what I got using a Fluke 187 multimeter, which should be fairly accurate. The rectifier tube is a Sovtek GZ34.

The power transformer secondary: 581 VAC (OK)
Center tap of output transformer primary: 346 VDC
EL84 Plates: 341 VDC
EL84 Screen Grids: 323 VDC
EL84 Cathodes: 10.8 VDC

Are the voltages too high?

[alvarezh]

George, no problem running 84's @341 vdc. The guys over at the Trainwreck side of the forum run them there constantly. I read of a bunch of different people running them @ 385 vdc.

The reason for your voltages being on the high side could most likely be because your bias is somewhat colder that what Nik (Ceriatone) used to get those voltage readings.

As you bias hotter, the B+ drops and vice-versa.

Enjoy your amp!

[George61]

For my next trick I am going to try the Paul Ruby buzz fix.
http://www.paulamps.com/18watterbuzz.ht ... Distortion
My problem is that I am not sure what zener diodes to use. The schematic is for an 18 watter and a single tube per side. The zener diode is a 9.1V .5W.

If the circuit is going to be applied to two tubes per side, as in a 36 watter, should I use a 1W zener? I know that a zener diode has a power range that it will work in. Rod Elliott states that it should be run from 10% to 80% of rated power. Being very weak in theory, I don't know how to calculte the approximate current the circuit would draw. Also, how would I check the circuit once that it is implemented in my amp?

Thanks in advanced to anyone who replies.

Solder on,
George