advice on tube trem circuit...

[HTH]

I've been messing with a single triode trem circuit in my cathode biased EL34 amp and would like some advice from you good chaps

The trem is lifted directly from the Matchless Tornado which is exacly like the Matchless Hurricane trem except that the speed pot is 500k on the former rather than 1M on the later; similarly, there is a 330k versus 470k resistor in the oscillator circuit.

Tornado... Schematics Download

Hurricane... Schematics Download

I'm wondering what affect those two changes have - at the minute I'd like the effect to be more noticeable as its a bit too subtle at present. Ideas/comments Anyone?

[fyl]

Not enough juice IMO. This kind of trem is designed for small bottles with lowish bias voltages - app. 8V for an EL84, it's too weak for large ones such as EL34's with 25V across the cathode R.

[HTH]

any ideas how to mod the circuit to work better with EL34s? - I could use 6V6s or 6L6s if they would work better, I'm not too fussed).

[fyl]

You need more output voltage. Try fiddling with pot values in the output section. Begin by going straight from the osc output cap to the 220K grid R's, thus bypassing the pot.

Using 6V6's or 6L6's won't change a lot: 6V6's bias at around 14V.

[HTH]

if I need more output voltage, would using a split plate load on the oscillator help and tap the signal from the junction of the two resistors?

also, as this is cathode bias, I'm not getting your reference to how various tubes bias up via the 220k grid splitters.

[d95err]

Can you measure the amplitude of the LFO? If you don't have an oscilloscope, it might be able to do if you turn the speed to max and use the AC reading on a multimeter. That would give an indication of if the signal voltage is big enough for the EL34s.

If the voltage is high enough, the problem could be not enough current. What value are your bias feed resistors that the intensity pot are connected to? On the Hurricane/Tornado they are 220k. A bigger value may decrease tremolo intensity.

[fyl]

if I need more output voltage, would using a split plate load on the oscillator help and tap the signal from the junction of the two resistors?

Won't be useful IMO.

also, as this is cathode bias, I'm not getting your reference to how various tubes bias up via the 220k grid splitters.

The app. bias voltage I quoted is read across the cathode resistor.

[HTH]

Can you measure the amplitude of the LFO? If you don't have an oscilloscope, it might be able to do if you turn the speed to max and use the AC reading on a multimeter. That would give an indication of if the signal voltage is big enough for the EL34s.

If the voltage is high enough, the problem could be not enough current. What value are your bias feed resistors that the intensity pot are connected to? On the Hurricane/Tornado they are 220k. A bigger value may decrease tremolo intensity.

Thanks for everyone's input so far, much appreciated.

I'll get the amp out tomorrow and measure the voltage at the (220k) bias splitters. I could lower the bias splitters to 82k like on some of the early 70s Marshalls if that would help - the depth/intensity pot is 250k.

[tubeswell]

if I need more output voltage, would using a split plate load on the oscillator help and tap the signal from the junction of the two resistors?

No that would decrease the swing. For more voltage on that circuit, I'd say you probably need to either lower the 22k supply resistor (to something like 11k or 5k or 1k) or connect the Plate resistor to the screen/choke power supply node, and then put in a bigger plate resistor (say 330k or 470k), then you might want to change all the RC combinations in the LFO; or the other hand, add a 100k-220k bleed out resistor if the voltage swing it too big. 2CW

[Wakculloch]

Could you inject the oscillator signal into the phase splitter, either at the grids or the cathode? Shirley this would take less drive and still provide the differential cancellation. Not tried this but i bet someone out there has.

[HTH]

the trouble is finding a trem circuit where the output stage is cathode biased.

could I just lift the trem stage from a Vibrolux 5E11 and just ground the bottom of the depth pot instead of connecting it to the -'ve bias supply since my OP stage is cathode biased????

http://www.ampwares.com/schematics/vibrolux_5e11.pdf

[tubeswell]

the trouble is finding a trem circuit where the output stage is cathode biased.

How about the GA17RVT? (albeit another low power tube amp)

[HTH]

How about the GA17RVT? (albeit another low power tube amp)

thanks for the suggestion - had a look at some Gibson schematics and the GA19RVT looks promising - cathode biased 6V6s and a single trem oscillator much like the one I'm using from the Matchless Tornado.