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Old 12th August 2010, 09:33 AM   #1
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Default Need help about an hybrid amp circuit

Hello

I've try to adapt a tube ltp input to an transistor amp schematic, I'm not sure but since I'm better with transistors I think my hybrid amp circuit may not work ok.

Anybody can tell my if my amp would work ok and which mods it could need ?

Thank

Bye

Gaetan
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Old 12th August 2010, 09:50 AM   #2
DF96 is offline DF96  England
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U1 has no grid bias - it needs a resistor to ground.
Q7 has no base bias - you need to feed it some current from somewhere. Maybe turn the whole solid-state section upside down then drive directly from U1 anode?
The input LTP will have very low gain due to the large cathode resistors.
What is the purpose of D2?
What is the purpose of R8? This will drop a lot of voltage, so will severely restrict the output swing.
Why does every device have a base resistor - is this to control parasitic oscillation?
Why is C12 drawn as an electrolytic?
Why is C14 present? In a normal amp this gives full DC feedback to stabilise the output offset, but here there is no forward DC path because of C1 so no DC stabilisation is possible.
What is the function of R6? Does it have to be exactly 332 or would 330 do?

I will stop there. I think you need to do a bit more reading. Most of the problems I have identified have nothing to do with valves.
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Old 12th August 2010, 10:09 AM   #3
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Quote:
Originally Posted by DF96 View Post
U1 has no grid bias - it needs a resistor to ground.
Q7 has no base bias - you need to feed it some current from somewhere. Maybe turn the whole solid-state section upside down then drive directly from U1 anode?
The input LTP will have very low gain due to the large cathode resistors.
What is the purpose of D2?
What is the purpose of R8? This will drop a lot of voltage, so will severely restrict the output swing.
Why does every device have a base resistor - is this to control parasitic oscillation?
Why is C12 drawn as an electrolytic?
Why is C14 present? In a normal amp this gives full DC feedback to stabilise the output offset, but here there is no forward DC path because of C1 so no DC stabilisation is possible.
What is the function of R6? Does it have to be exactly 332 or would 330 do?

I will stop there. I think you need to do a bit more reading. Most of the problems I have identified have nothing to do with valves.
Hello

I've just adapt a tube input to a working amp schematic and keeped all the ss part as is, it's a first try.

My schematic software alway put capacitor as electrolytic, so C12 are drawn as electrolytic.

I can ad a 500K grid bias resistor from U1 grid to ground.

I can reduce the cathodes resistors to 150R

D2 are to keep the voltage of the VAS a bit more stable wen the output take more curent.

R8, R11 and C9 are the boot-strap of the amp.

R6 was just to limit the current to U2, and 332R was a resistor that I have in my parts bin.

Thank

Bye

Gaetan

Last edited by gaetan8888; 12th August 2010 at 10:14 AM.
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Old 12th August 2010, 10:29 AM   #4
DF96 is offline DF96  England
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Sorry, I did not spot that C9 is a bootstrap. It makes more sense now.

R6 won't do anything useful so you can omit it. Base bias for Q7 and DC stability are the remaining issues you need to look at.

Why did you choose 12AX7 for the input? If you used ECC88 then you might be able to run it from the 35V supply, and use DC coupling to an inverted circuit.
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Old 12th August 2010, 10:39 AM   #5
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DF 96 has his comments on your circuit, but the GLARING fault he has not mentioned is that your input "LTP" isn't a LTP at all. At least, it isn't to my eyes, but I am not familiar with simulation circuit drawings.

You show the input tube to have 1k cathode Rs, that's not a big problem IMO, but they are hooked to a minus 120V supply - which is a big problem, as this will NOT WORK.

A LTP needs a CCS in the cathodes, if the 1k's were to join a CCS, which then joined the minus 120V supply, then it would work, but right now you will get NOTHING out of it.

And DF96's suggestion to use a 6DJ8 input tube is a good one, but they DO NOT run very well on 35V, 100V is more like it.

Regards, Allen
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Old 12th August 2010, 11:20 AM   #6
Arnulf is offline Arnulf  Europe
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What's the point of running a tube input section, followed by further sandy amplification section and then apply dozens of dB of NFB to the whole concoction ? If you don't like the effect tube has on sound, skip tubes alltogether and the whole thing will work just as well.

If you want tube to imprint its characteristic distortion on the signal, follow it by unity gain buffer and leave it outside the feedback loop.
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Old 12th August 2010, 01:42 PM   #7
teemuk is offline teemuk  Finland
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AC coupling the voltage amp is a big no no unless you implement a way to keep the output's DC offset on leash. A DC servo perhaps?

If you want to get a "tubey" sound then driving a single-ended transistor voltage amp with a tube LTP input is a pretty weak scheme for that.

I'd rather try to sum the opposite phase signals you can acquire with the LTP circuit and then voltage amplify - or if amplitude is enough, simply buffer - that signal.

Quote:
... the GLARING fault he has not mentioned is that your input "LTP" isn't a LTP at all.
It is LTP allright ...or at least I think the idea was that it should be one ...but an important issue and error is that it's missing the resistor that would isolate the node that sums the cathode signals from the low rail. Effectively, the cathode of the "input" tube can't drive the cathode of the other tube in the pair, that in a correct LTP would be configured as common grid amp. So, as is shown in the schematic, the circuit won't work as an LTP and the other half is basically doing nothing. Like it was mentioned already, this circuit has a lot of issues that are fundamentally quite incorrect to begin with.

Also, this supposed-to-be LTP is not configured as a phase inverter. In fact, it simply omits the use of the non-inverting output and proceeds to drive a single-ended voltage amp with the output signal of the inverting amp. The other half is there just for overall error correction providing an inverting input. This scheme is much used in SS amps because an LTP circuit can have a great amount of open loop gain, which is then tamed with negative feedback to reduce distortion by great amounts. Because of those major differences the circuit will behave very differently than the LTP used in tube amps as a phase inverter for a push-pull stage.

Gaetan, you may wish to study German patent #103 26 581 (describing Hughes & Kettner's "DynaValve" circuit). It's an implementation of a triode LTP driving a solid-state power section. It also includes a grid bias simulation and provides a pretty realistic emulation of a typical tube push-pull power amp.

Last edited by teemuk; 12th August 2010 at 02:12 PM.
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Old 12th August 2010, 06:15 PM   #8
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Hello

I'm a real newbe in tube amp

I do all my amps with transistors and I did want to try tube hybride without using the usual mosfet output.

I only have 12AX7 tubes.

I admit that I've try to apply SS amp LTP to tube without lot of mods.

Maby I should use a SRPP input (it could be more simple) ?

I will check that patent as suggested.

Thank

Bye

Gaetan

Last edited by gaetan8888; 12th August 2010 at 06:19 PM.
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Old 12th August 2010, 06:38 PM   #9
Arnulf is offline Arnulf  Europe
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There is no such thing as "the usual MOSFET output". In case you're referring to "Zenkido" and similar arrangements, well, they are about as usual as any other DIY idea.

What you shouldn't be doing is goofing with parts of circuits you don't understand, crazy-glued together in an attempt to make something useful.

You say you've got 12AX7. These are very nice input section tubes, you can easily get 50x amplification from a single section. 50x voltage amplification translates to 20-40W per channel with line input and a suitable unity gain stage after the tubes. By using a single 12AX7 (one section per channel) you can get the "tube sound" without any impairment from NFB and all the power you'll ever need from the sandy section.

If I were you I'd put together a simple common cathode stage (2K cathode resistor bypassed by 47 uF or thereabouts, 100-200K anode resistor, B+ in the range of 200-300V) and couple it to a power buffer. Power buffer can be something as simple as BC550 (or similar) emitter follower input, working into CCS, driving PP darlingtons.
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Old 12th August 2010, 06:50 PM   #10
DF96 is offline DF96  England
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You can't simply swap LTP for SRPP, they are quite different circuits for quite different purposes. As I said in my first post, I think you need to do some more reading. In particular, look at AC coupling, DC coupling and bias arrangements. Then you need to decide what you are trying to achieve.

In theory you could replace the BJT LTP at the input of an amp with a valve LTP, but it would be a poor fit so why do it? If you really want to do this then invert the whole solid-state section as I suggested. This is because you can't buy positronic valves, but you can buy both polarities of BJT.
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