Top Boost type Preamp Voltages

[sdp1234]

I'm working on an amp that  has a top boost type preamp with a mosfet as the CF.  I think my voltages on the second stage seem a bit low.

V1
B+ 295
1- 149
3- .97

6- 96
8- .73

attached is the schematic that is close.  Does 96 seem low (bottom of 100K plate resistor)

[HotBluePlates]

The MOSFET appears to be drawing gate current through the 100k plate load for the 2nd stage, which is why the plate voltage is lower than expected.

A basic assumption in tube electronics is that under most conditions, the grid of a following stage (the cathode follower in your schematic) represents a very high impedance and therefore draws almost zero current. That is not true in solid-state electronics, and is why we can't just replace a tube with a transistor without some changes.

You may need to add a coupling cap between the 2nd stage and your MOSFET source-follower (the FET version of a cathode follower). The MOSFET will now need 2 resistors between B+ and ground to define its bias. However, I don't know enough about solid-state circuits to recommend values to you. Perhaps someone who knows more will chime in.

[sdp1234]

Kind of makes sense but I've seen others use the IRF820 fed directly through a 220 ohm resistor.  I wonder is this was happening for them?  Anyone else seen this?  If so, should I have the coupling cap?

[HotBluePlates]

It won't fix the preamp, but simply disconnect the MOSFET gate and see if the 2nd stage plate voltage lands where it should be. That will tell the tale.

[sdp1234]

That takes the voltage to 204.  It really doesn't sound bad.  Maybe a little less clean and not quite as much volume as I'd expect.  I want to get this straight before I tweak anything else.

[sdp1234]

Wow... I had the mosfet backwards.  Gate and Source switched.  Voltages are in line now.  Have to wait for my son to test the tone.

Thanks for helping a moron.

[HotBluePlates]

Well, I think you figured that one out yourself. I don't know squat about MOSFETs, so I just knew where to find a problem based on the tube voltages.

[rock_mumbles]

So any more info on this one?  tone etc?

I'm thinking of doing a similar thing ...

What did you use for the source resistor, a 100k from the MOSFET Follies papers or the 56k from the Vox schematic?

[Boots Deville]

I've got one wired as follows:


This goes into a 2 x EL84 power amp, and I've built the same preamp with a tube CF into a 4 x 6V6 power amp, and they both sound great to my ears.  I haven't done any "scientific" tests or side by side comparisons, but I'll definitely use the MOSFET CF again in the future.

[sdp1234]

Something had gone wrong with my output transformer.  I expect to order another real soon.  Sorry but I can't comment on the tone yet.  It has sounded great at times and also like crap at times so I still have work to do.  I believe my problems may be with lead dress and layout because this amp has been three different things and I didn't start from scratch.  Probably would have been better to gut it and start from the begining.

[sdp1234]

Boots,

Thanks for the schematic with voltages.  My preamp voltages are much higher.  Are you still able to get good cleans with the lower preamp voltages? 

[Boots Deville]

Boots,

Thanks for the schematic with voltages.  My preamp voltages are much higher.  Are you still able to get good cleans with the lower preamp voltages? 

No problem, sdp.  My "inspiration" wasn't the Top Boost channel, but instead the Trainwreck Rocket, which was itself inspired by the Top Boost channel.  I dunno what voltages Vox had there, but the Rocket had even lower supply voltage to V1 than I do - around 185 is what I've seen published.

But to answer your question, yes, great cleans.  This is the clean channel of a two channel amp.

[tubeswell]

I too have used DC-coupled IRF820s as source followers in a few tremolo circuits with 100-200R resistor in series with the gate (and a 12V Zener slapped between the source and the gate for extra protection). These mosfets can be used as direct subs for a triode stage because of their 500V rating.

Edit - And here (thanks to Mike Donovan) is a circuit featuring even more elaborate use of IRF820s as subs for triode stages.

[rock_mumbles]

Thanks!  One thing I just noticed on the AC-30 schematics I've seen they have a 1.5k||22uf shared cathode for the Normal and Bright channels, then like in the first post the 'simplified ' Top-Boost' only preamp has a single triode with 1.5k||22uf cathode ???

Anyway ... I've read where people are afraid the MOSFET will ruin the tube amp tone, I want to build a pp 2 x EL84 with this sort of preamp.

[Boots Deville]

Are you the same "Rock_Mumbles" from the land of 18Watt???

I was just over there yesterday for the first time in a long while and stumbled on the Superlite IIB threads where you'd apparently added a MOSFET follower with positive results.  I have a set of 18W transformers on the shelf and was inspired enough to draw up a layout for such a beast last night.  I still have a few projects to get off the bench before I can get drilling.

It might be kinda cool to run this top boost channel preamp along side the Superlite going into the "B-side" of the LTPI, since there's no NFB.

[rock_mumbles]

Same rock_mumbles ...

A friend wants me to build him an amp so I'm thinking along similar lines ... channel one 18 watt LiteIIb with channel two a TopBoost channel w/ source follower.
With the LiteIIb being a single stage and TB channel two stages when connected to the opposite sides of the LTPI they are in phase.

I want input from other people is why I asked, I've used MOSFET source followers in three amps now and am pleased with the results.

I haven't built it but I edited an existing Superlite layout to update the circuit and ad the source follower ... http://rh-tech.org/public/18-Watt/Superlite_II_w_SS-rectifier_MOSFET-source-follower_layout-b.jpg

[sdp1234]

This thing is killing me!!!  I've had it finished several weeks ago.  Sounded OK but not right.  Found that the Mosfet was in backwards.  After changing the Mosfet it sounded like crap.  Buzzy distortion at all volumes, while testing the volume dropped drastically.  After changing many components finally replaced the OT and the Volume came back although it still sounds bad.

I'm thinking maybe some type of oscillation killed the OT?
Seems like the problem may be in the preamp because I've taken the output from the preamp into a SE power amp and it still sounds bad.
Ive removed the cathode caps from the first two stages to reduce gain still this bad distortion.  I realize that it is usually caused by faulty layout or bad lead dress but I can't find what is causing it.  Is there something special that I have to do with the Mosfet?
 

[sdp1234]

Rebuild my listening amp with a volume control.  Sounds good at the output of the first volume pot.  Clean and clear with no distortion.  Sounds bad the output of the tone stack.  Not real good at the output of the second stage (pin 6 of v1).  Fuzzy distortion on top of the sound.  Disconnected the Mosfet CF.  Still sounds bad at the output of the second stage.

This is pretty basic.  The first and second stage are laid out almost the same as I've done 4 or 5 other amps.  I'm going to pull out the components and check their values and the connections.

At least I know where it is starting.

[tubeswell]

I'm going to pull out the components and check their values and the connections.

At least I know where it is starting.

Before you do that just check all the DC idle voltages at:

1) All the filter caps (+ve end) supply nodes; and
2) All the plates, screens, grids, and cathodes of all the tubes

and check the VAC supply (across each PT winding)

[sdp1234]

I disconnected the 220ohm resistor that goes to the gate of the mosfet.  Injected a 200mv 800 hz signal into the input.  Looks good at the output of stage one.  Stage 2 distorts slightly when volume is cranked all the way.  Sounds good through listening amp at the output of stage one.  Sounds ok at the output of stage 2.  a little bit of ghosting on top of some notes played hard.

When the signal is injected into the input I'm getting an 800 hz 22v PP at the gate and drain of the mosfet when viewed on the scope. The source has a 60v pp 800 hz.  These readings are without the 200 ohm so there should be no signal.  The amplitude changes with the volume.  The frequency follows my signal generator if I increase  or decrease.  Is this some sort of bleed through?

[sdp1234]

Ok... Finally making progress.  Looks like the layout is giving me problems.  I've moved a few things around and added a lot of shielded cable and it is getting much better.  I will not be able to do any more tweaking until the OT comes. (I don't want to install the temporary OT and have to take it out in a week when the real one comes.  Some of my instability now is probably from the temp OT that is just hanging on the side.

Why are the TopBoost Vox\Express\Rocket type amps so difficult with layout, squealing and oscillations?  I've build several High Octane type preamps with multiple channels and never had to use more than one section of shielded cable.  I've had a few that looked like complete rats nests but they have no problems compared to this type circuit.  Seems like 2 gain stages through the CF to the PI would not be more gain than the HO style?  Guess I better try laying out one of these with a known working layout.