Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on October 04, 2013, 03:47:46 pm
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Has anyone experience in installation of tremolo in an SE amp with VVR.
The tremolo oscillator should be connected to the cathode of the second amplification stage.
What would be the best option to implement the amp:
- Regulate the whole amp including tremolo
Would the tremolo (oscillator) work with lower voltage supply? - Requlate the whole amp excluding the tremolo
- Regulate the power tube only and supply the preamp and tremolo with high voltage
/Leevi
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This would be my starting place. I haven't built this but have used VVR on a number of amps.
I attached an editable SCH version, so you can (hopefully) come back to the forum showing what you ultimately did.
With respect, Tubenit
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Thanks for the schematic Tubenit,
This is about the circuit I'm planning to implement.
One difference is that I thought to replace the tone stack by Princeton's tone control in order to minimize the signal loss.
Then I will add an optional power tube EL84 and a switch that can be used for selection of either 6V6 or EL84.
Your proposal seems to be
Regulate the power tube only and supply the preamp and tremolo with high voltage
I'm just wondering if a master vol is needed since the preamp is not scaled?
/Leevi
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I'm just wondering if a master vol is needed since the preamp is not scaled?
Yes or you'll over drive the power tube as you turn down the B+ voltage on it. It takes less and less preamp drive signal to drive the power tube to full output as you turn down the B+ dcv.
Brad :icon_biggrin:
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Based on the discussion I attached the schematic.
Please comment if you have any.
/Leevi
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I got the amp wired. I ended up to solution where tremolo works only when the
VVR has been bypassed. I tried the tremolo with VVR but it didn't work with dropped B+.
Scaling the power amp only was not a option for me.
See the updated schematic.
/Leevi
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Tremolo oscillators are kinda finicky. However, if you VVR the entire amp, EXCLUDING the trem circuit, it should work. Trem might be too strong when the B+ is turned down, but you should be able to compensate with the intensity control.
I would think that simply connecting the bottom of that 2.2K that feeds node D directly to the right side of the choke would do it. I'm just thinking. I've never messed with VVR so I can't offer any guarantees.
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I have done a couple of these , very similar to the Hall VVR , the only way I can see this working is as Sluckey has suggested you have to regulate the voltage either directly off the recto tube or the choke I recommend the choke , IMHO the entire amp is not scaled then the voltage offset will not allow the tremolo to operate correctly.
move the 1n4007 diode that feeds nodes C and T to the same point that feeds node A the input side of the choke , your tremolo circuit isn't being scaled , I think that may be the problem.
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EXCLUDING the trem circuit, it should work. Trem might be too strong when the B+ is turned down, but you should be able to compensate with the intensity control.
I tried it:
It is too strong and you can partly control it but I didn't find it usable at all.
Furthermore when the tremolo is turned off the voltage level on node E is too high which
blocks the second amplification stage on lower B+. In this case the tremolo should be totally disconnected
from node E or its voltage supply should be turned off.
move the 1n4007 diode that feeds nodes C and T to the same point that feeds node A the input side of the choke , your tremolo circuit isn't being scaled , I think that may be the problem.
Sorry, I didn't get that. Are you referring to tubenit's picture?
/Leevi
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Maybe consider a tremolo scheme like the Marshall 1974 18 watt amp. The trem modulates the first preamp tube. Preamp and trem oscillator are in the same bottle and are powered by the same B+ node. So, VVR everything except that B+ node and all those trem issues should go away.
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So, VVR everything except that B+ node and all those trem issues should go away.
Of course that would be an option. Where can I find the schematic?
/Leevi
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what I am saying is the problem as far as I see it anyway , is that your scaling (lowering the voltage) to everything except the tremolo circuit , it's over powering the rest of your amp . If it was sitting on my bench I would place a single diode 1n4007 would probably be fine of the output of your recto tube go into the VVR circuit then feed your filter caps and dropping resistors from there . Not to say you can't solve the problem the way the other guys on here are suggesting or some other way , but that looks like a quick and easy fix to me .
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I've attached the original schematic. But here's a redrawn schematic that's easier to look at. And there are plenty of Marshall 18 watt schematics all over the internet.
http://home.comcast.net/~seluckey/amps/18w/18w.pdf (http://home.comcast.net/~seluckey/amps/18w/18w.pdf)
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I would place a single diode 1n4007 would probably be fine of the output of your recto tube go into the VVR circuit then feed your filter caps and dropping resistors from there
If you mean that I should scale the voltage of the tremolo circuit I have tested that.
It worked partly with higher voltage values of B+ but when I decreased the voltage tremolo stopped working. Anyway that was the best alternative of the combinations of tremolo and VVR.
/Leevi
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I see well sorry brother , this would be a problem for the wise ones on here , obviously of which I am not lol
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A LFO with a 3-stage CR network has to have a gain of at least 29 to oscillate according to http://www.valvewizard.co.uk/trem1.html (http://www.valvewizard.co.uk/trem1.html)
But then he suggests that if you use a 4-stage CR network, you can get the gain down to 19 and it will work, so maybe you could try that? (seeing as how gain is a product of a number of aspects - including HT voltage), or just ensure that the supply voltage for the LFO is independent of the VVR (like Steve originally suggested)?
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Thanks Tubeswell, this is a very interesting article about tremolo oscillators.
/Leevi