bias modulating trem and paraphrase phase inverter

[funkmeblue]

anyone have any experience adding a bias modulating trem circuit to an amp with a paraphrase phase inverter such as a fender 5c5 or an old valco? I only have one extra triode to use and could not find any examples out there. I am guessing there is a reason not to do it. Thanks in advance.

[gingertube]

You would need to do some rewiring around the phase splitter to accommodate it but it could be done. You need to separate the grid leak function from the signal divider to the inverting triode.

Trying to think of what might be a suitable example to look at.
Try this one:
http://unofficialampeg.com/schematics/echotwn2.gif
Cheers,
Ian

[randalp3000]

I did it on a tweed deluxe kit a long time ago. I copied a gibson falcon ga-19rvt with cathode biased output tubes. It worked great. Or princeton reverb with fixed bias output.

[funkmeblue]

ya I was concerned about the grid leak function.....I was actually planning on using the ampeg trem circuit and 6sl7's....I will have to study that schematic a bit, why did it have to be stereo lol

[gingertube]

ya I was concerned about the grid leak function.....I was actually planning on using the ampeg trem circuit and 6sl7's....I will have to study that schematic a bit, why did it have to be stereo lol

Note it is 2 completely seperate amps in 1 chassis. (One to use and One spare - HoHo) Did a restoration on a Echotwin 1B about 3 months ago. If going for the 6SL7 use the Echotwin 2 circuit values for cathode resistor etc. on the oscillator. I found that at slowest rate setting on the speed pot the oscillator was wanting to fade away (along with the tremlo effect). Fitting the cathode resistor values from the Echotwin 2 sorted that.

Cheers,
Ian

[funkmeblue]

I am worried that the ampeg phase inverter in that schematic is not going to give me the "same" sound as the fender or valco....any opinions on that? I was also wondering if you know of any single triode trem circuits that work on the cathode voltage on maybe the preamp or cathodes of the phase inverter.

[gingertube]

I would'nt worry about the "different sound". It will not be very different if at all.

A single triode solution to tremlo via a preamp tube is probably not going to work.
Here is the tremlo preamp of a vintage Oz Amp (Moody GA40)
http://www.ozvalveamps.org/moody/moodyg ... olocct.gif
and here is the oscillator driver
http://www.ozvalveamps.org/moody/moodyg ... osccct.gif

Note that the Oscillator signal is buffered. That is because it has to drive into the low impedance presented by the cathode circuit of the preamp tube. The cathode resistor here is ONLY 1K and worse, this is in parallel with the impedance "looking into" the tube cathode ( approx 1/gm) which for a 12AX7 or 6SL7 will be around 600-700 Ohms. 1K parallel 700 Ohms, call it 400 Ohms.
The buffer on the oscilator signal is absolutely required to drive that 400'ish Ohms. In fact a far better buffer than that shown would be an advantage. The problem here is the requirement for 2 triodes.

Stick with the first idea.

Cheers,
Ian

[funkmeblue]

do you think there would be a problem increasing the plate resistors from 47k to 100k and also the 200 r resistor on the cathodes is just there for negative feedback loop, right? thanks again

[cbass]

Not sure about wiggling the paraphrase inverter with one triode but I have found bias modulated tremolo on cathode biased amps to suck better off wiggling preamp cathodes or signal will take two of triodes though

[gingertube]

For guys following the thread - the OP is talking about the Ampeg ET-2B schematic linked above.

The 200R is as you say for feedback and can be simply left out.

Looking at the load on the PI there is 470K parallel 270K = 170K.

6SL7 internal rp is about 44K
Zout (with bypassed cathode resistor) is approx 44K parallel that anode load resistor, say 44K||47K = 22K with the 47K and 44K||100K = 30K with the 100Ks. That will make a Rodents Hind Quarters difference.
So that change can be made too. Watch the idle current, you don't want excessive voltage drop across the 100K, you may need to take the cathode resistor from 1K up to 1K8 or 2K2.
What to look for here is that you want the anodes to sit at about 2/3 of the High Voltage Supply.

On the post above, I think this will depend upon the size of the output tubes cathode bypass capacitor to some degree. The Ampeg with just 20uF was fine. When I did the restoration on the Echotwin 1B I made sure that I replaced the old 20uF (part of a multicap can) with a modern 22uF rather than a larger value.

Cheers,
Ian

[funkmeblue]

so it would be better with a lower value cap on the cathode resistor or better yet none

[gingertube]

Sorry I'm passed my experience here.

I did some thinking as follows:
Works by modulating the output tube bias
therefore we don't want the cathode voltage following the tremelo signal applied to the grids.
Therefore we would want the cathode bias resistor + bypass capacitor to be such that the timeconstant (R x C) is greater than say 3 to 5 times equivalent timconstant for the lowest tremelo oscillator frequency.

So lets see what happens when we plug in some values:
lowest tremelo frequency = 1 Hz
Equivalent timeconstant = 1 / 2 pi F = 160 millseconds

want cathode bias R and C to be 5 times that = 160 x 5 = 800 ms

On the Ampeg the R is 140 Ohms so minimum C = 0.8 / 140 = 6,000uF

Just looked down to see if there was a hole in my foot.

HELP -
- Is cbass right when he says it doesn't work too well with cathode biased amps, it seemed to work OK on that Ampeg I restored?
- Have I made a basic BAD ASSumption in the maths treatment? (This is the usual problem when things don't seem to make sense)
Have I screwed up the math?

So sorry funk - we need some more input from someone else.

Cheers,
Ian

May the mass x acceleration be with you.

[funkmeblue]

I restored a 60's lectrolab with cathodyne phase inverter and cathode bias el84 output and it sounds glorious, but I want that nasty old paraphrase, thanks for all your help, looks like a modified ampeg inverter and trem circuit may be added to the mix

[cbass]

[quote="gingertube"]

HELP -
- Is cbass right when he says it doesn't work too well with cathode biased amps, it seemed to work OK on that Ampeg I restored?
- Have I made a basic BAD ASSumption in the maths treatment? (This is the usual problem when things don't seem to make sense)
Have I screwed up the math?


Didn't mean to confuse things I was just saying that when I tried modulating the output tubes It didn't sound as lush as my fixed bias amps.
my assumption is that cathode biased amps are typically biased hotter so the signal isn't cut as much
You obviously know a lot more than me so I'll just bow out