One tube oscillator

[Leevi]

Has anyone tried an oscillator where the triodes are parallel connected
and does that give any benefit when used in the Trem-o-nator circuit?


/Leevi

[tubeswell]

No. But. Go for your life.

[sluckey]

Looks like a good way to waste a triode. 

The single triode oscillator puts out over 100Vrms signal on the plate. Do you think you need more?

[Leevi]

Thank you for the replies. It's then  better to leave the second triode unwired.


/Leevi

[tubeswell]

Or use the 2nd triode as a CF for a low impedance buffer between the LFO and the following load (e.g , for bias vary trem applied to output tube grids)


Or if it’s a push pull amp with a cathodyne PI, use the spare triode as the cathodyne (like a BFPR) and gain another preamp triode to use elsewhere in the preamp. Cathodyne is good use of spare triode because the output is balanced and any thumping crosstalk from the nearby LFO triode gets ‘cancelled out’.

[sluckey]

I'm not saying one way is better than the other. I'm simply saying that parallel triodes are not necessary and don't offer any benefits to the trem-o-nator or any other tremolo phase shift oscillator.

And why does there even need to be a second triode? You could just as easily use a 6AV6 which is much cheaper than a 12AX7.

[Esquirefreak]

+1 for using 6AV6 (or other heptodes) for LFO. High quality tubes, but dirt cheap!

[Leevi]

The circuit is ab763 with 6L6s (~Super Reverb) and I'm looking for another type of tremolo in order to avoid thumping issues. Tremonator fits good to this circuit with 50K reverse audio pot.


/Leevi

[astronomicum]

It's then  better to leave the second triode unwired.

Figured I would kill two birds with this post: Circuit review and offering the OP a 2 triode alternative.

Quick back story. The original tremolo circuit for a few of the Univox models uses a back-to-back 12AU7 triode as a variable resistor to shunt the signal. It works but it is prone to hum, and a lot of it if a noisy tube is used. I have been able to minimize hum on previous amps by some tube rolling. Vintage Mullards are the quietist. Unfortunately, I have run out of the lowest noise Mullards I have and was not satisfied with the hum in the U-1221 I am restoring.

I started working on a circuit modification a couple weeks ago. I started with the Tremonator as an alternative. It worked well, reduced hum to negligible levels, and eliminated the 12AU7 entirely, but there was a loss of bass response due to the high pass filter set up by the intensity pot (in this circuit anyway), and I was not entirely happy with the sound of the VTL5C1 (will come back to that).

I decided to try separating the LFO from the Vactrol Driver using a second triode since I had an empty socket available. This allowed me to remove the intensity pot from the signal path which made a notable improvement in tone. I only slightly modified the original LFO to slow it down a little (I personal like the slower speed). I had to test a few cathode resistor values until the Vactrol/LED was completely off so there was no loss of volume when the trem is off. Settled on a 4K3.

Coming back to the VTL5C1, I first tried modifying the voltage divider to the Driver grid, and the Driver plate resistor to see if I could smooth out the sound a bit but was not very successful. I then decided to bench test some homemade alternatives to the VTL5C1 from Red LEDs and commercially available 5mm photoresistors of various values (GL5506, GL5516, and GL5528). They were easy to make, worked well, and I ended up preferring the sound of the 5528. It is a smoother transition and does not completely kill the signal at the Driver’s maximum current. I conducted current vs. resistance tests to graph the response of the 5528 vs. VTL5C1 to make sure I was not “hearing things”. The plots confirmed what I was hearing. I want to plot them in XL when I get a chance and will post if anyone is interested.

I would appreciate any feedback on the circuit attached before I modify the amp. It is currently on the breadboard so it would be easy for me to try/confirm any suggested modification. Schematics of the original and modified circuits is attached.

This may also give the OP some things to try.

[Leevi]

It looks a very interesting alterative. How strong is the tremolo effect if you compare it with the tradiotional ab763 tremolo? How about thumping?


/Leevi

[astronomicum]

Just realized I did not correct the original schematic speed control error in the modified schematic. See attached.

[astronomicum]

It looks a very interesting alterative. How strong is the tremolo effect if you compare it with the traditional ab763 tremolo? How about thumping?

Just like the Tremonator, which this is largely based, there is no thumping. With the Amp volume and Tremolo intensities at max, only a slight wavering in the noise floor is discernable.

I have limited experience with the ab763 Tremolo, therefore, I can not offer any comparisons.