36 Watt working, sort of.

stevesuk · Post by **stevesuk** » Tue 03/23/04 3:53 am

I had another go at my 36 watt last night and made a couple of changes. I took the grid resistors off pin 1 (used as a terminal) off the EL84s and connected them straight to the board I also swopped the OT leads, one of these was causing a very loud screaching noise even without anything plugged in. As I did both at the same time I don't know which change cured the screaching.
It now sort of works on both channels.

There is a remaining issue. Gentle strumming at any volume level is fine, although obviously uninspiring. If I hit the strings hard or play a power chord it crackles badly and the speaker ' tizzes ' a bit like loudly torn paper.

I haven't used a choke, although I have one ready. It was too late last night to try the choke, so I'll try it tonight. Perhaps the power supply needs a choke to cope with the load of high demand ? I thought a choke was mainly for hum and buzz suppression. But am I right in thinking it holds a reservoir of power for sudden demands as well ? It is very quiet with just a bit of hiss at full volume. I think that is probably down to following Jersey Aarons earthing and layout exactly.
Keep you posted on progress. Like Graydon and Richie, don't want to post any pics or circuits until it is sorted. Nearly there !!
Steve UK

Post by **zaphod_phil** » Tue 03/23/04 5:01 am

The choke is used to help regulate the screen voltages, so they don't drop when the amp is cranked. Using a choke supply to the screens will reduce the sag in the amplifier. What you are encountering is probably either some parasitic oscillation creeping in, or the need to apply the Paul Ruby buzz mod.

Ballz · Post by **Ballz** » Tue 03/23/04 6:10 am

I think this might be a severe case of signal overload - either overload of the PI itself or overload of the power tube grids. Missing grid leak resistor? Missing something else?

Cold solder joint in the output section?

One more check-up of everything is going to clear it up, I am sure. You have a scope so you can see where it happens?

Cheers /Richard

Post by **Graydon** » Tue 03/23/04 9:27 am

Steve,

What you describe sounds a little like a problem I first encountered with my TMB. The signal coming from the TMB circuit was too hot for the phase invertor and I had to attenuate it (it looks sorta like a master volume but don't tell anyone!

) before the preamp circuit hit the phase invertor. In my case, the power amp worked fine so the amp would sound good until the note started to fade away and then there would be this nasty crackling sound. In your case, your phase invertor values and power tube parts values might need to be tweaked to accet the signal coming from the preamp. BTW, I am using 1500 ohm grid stoppers right now per the AC30 schematic and they are connected to pin 1 (as a terminal) on the EL84s

On my 36W, I did some tweaking over the weekend and then again yesterday because I found my problem with the treble control not working on the TMB circuit(I had soldered the feed to the treble pot to the wrong eyelet on the board duh!) Now, the preamp works properly and the power amp is pretty good but if I try to play palm-muted power chords, there is too much hash that extends into the muted space beween the chords. I think I still need to do some tweaking of the parts in the power section. I think it is still biased too cold so that will be my next step. It sounds good though. It responds very much like my 18W but it has a bigger bottom end and more "presence" It is only a couple dB louder which I think is about what I expected. We're only doubling the power after all. It is beginning to feel like my old Budda Stringmaster which is also not too surprising since it is very similar.

Im still using the twin 18W OTs BTW although I did get some custom 36W OTs from Heyboer on Friday. I'll try one of those soon too.

stevesuk · Post by **stevesuk** » Tue 03/23/04 11:27 am

Thanks Graydon,

I'll change the grid stoppers from 820 ohms to 1.5K. They were a pig to install as well. What PI values do you suggest trying? I used the values in JA/Richie version 1.5 MV TMB drawing.

It is definitely a loud crackling sound.

Steve UK

Post by **Graydon** » Tue 03/23/04 11:52 am

Steve,

Right now the grid stoppers are 1500. The cathode is 100 but I need to change that as I am only getting 9v. Voxx used a 50 there. I want to try to get that up to around 12v like the 18W and see what happens.

I forget what I had on the other pins of the power tubes but most of the rest of the PI is stock 18W.

Post by **zaphod_phil** » Tue 03/23/04 12:26 pm

stevesuk wrote:What PI values do you suggest trying?... It is definitely a loud crackling sound.

In theory there's no reason the PI shouldn't have the standard 18W values (possibly with the 47k tail resistor). The 470ks on the grid will help ensure the PI doesn't get overloaded with too high of a signal level coming in. That loud cracking noise can sometimes be arc over in the EL84s when cranking, if the anode voltage and/or load impedance are too high. What voltages have you got there, and what load impedance are you actually running? Have you checked that you've got the right OT connections for 4k load?

stevesuk · Post by **stevesuk** » Tue 03/23/04 1:45 pm

Phil, Graydon,
I have just changed the grid stoppers to 1.5K no difference, perhaps worse.

The tail resistor is 56k.

The hammond I have used is an awkward thing to wire on the output, it seems you can have either 4 + 8 or 16ohms on its own. I have gone for the 4 and 8. On the primary I have gone for P and P with centre tap for B+. I have taped up the two Sg's ? Diagrams attached.

It does sound violent enough to be flashover.

Steves UK

stevesuk · Post by **stevesuk** » Tue 03/23/04 2:43 pm

I have just tested the voltages and put the 10 Henry choke in.

The mains is incredibly high today at 248 volts ( the declared voltage is 230V)

The HV from the 36w heyboer is 311 Volts.

The B+ is 349 volts

The heater voltages are close on spec.

Steve UK

Post by **Graydon** » Tue 03/23/04 4:09 pm

Steve,

That OT looks similar to the Radiospares OT pinout. They have two separate windings. They are both 8 ohms but one is tapped at 4 phms. I run the two 8-ohm windings in series for 16ohms. I hope this is correct. This is how my PA20 is wired from Marshall with the RS OT. I think I have figured out a way to wire it up to a 4P3T rotary impedance selector so that all three settings are switchable. I need to try it but I have to order the switch first.

Post by **zaphod_phil** » Tue 03/23/04 6:37 pm

stevesuk wrote:I have just tested the voltages and put the 10 Henry choke in.

Put it in where? I doubt that the choke is your answer, unless of course it's a big choke that you're regulating the main B+ through, to bring the B+ voltage down to a sensible level - although it will also reduce or eliminate your power supply sag. If your load impedance is too high, that can also cause excessive "flywheel" voltages to appear on the EL84s, which may cause the arc-over to happen. Also, some modern production valves can be more prone to arc-over, due to manufacturing quality problems.

stevesuk · Post by **stevesuk** » Wed 03/24/04 3:08 am

Phil,
I am using NOS Brimar and Mullard tubes except for a JJ GZ34.

What do you think of the voltages ? they seem a little high but not excessive ?

I haven't really got anything else to try now, other than poking around. Check the wiring for the tenth time and redo all the solder joints.

Steve UK

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36 Watt working, sort of.

36 Watt working, sort of.