Fender Hot Rod Deville "more drive" issue

[CornElvis]

Hi All,

I'm new to the forum, but not new to tube amps and I repaired a couple of these amps already and I know they are kind of dreadful to service, but anyway.
The thing is everything works great until you push the "more drive" button, it gives me a tone, with all the knobs turned down and the master a bit open, it gives a tone around F# on a normal tuned guitar.
I checked most of the check points from the scheme and all seem fine.

There is a little history on this Amp, it came in, because the guy had pushed in his 6L6's incorrectly (he must have pushed really hard to do that but.....) and the amp started to smoke he said. That was true as some components were burned out. I replaced all the bad components and all measure fine. I re-soldered the two cement resistors, but to no avail.

I attached a sound file (in the zip there is a MP4) to make it a bit more clear.
This is the first time I hear this in these amps (won't be the last I guess) but maybe you guys heard this before and know where to look for.

Any help would be very appreciated

[Jazz P Bass]

So the 'more drive' when selected makes an F# tone by itself?
With or without a guitar plugged into the input?
There is not all that much to the 'more drive' circuit.
U3-A (TP36) provides +16Vdc to the gates of Q1 & Q2 (JFet J111).
J111 is an N channel jfet, meaning they are always 'on' & need a gate signal to turn them 'off'.
The 'off' signal is provided by U3-A.
When Q1 & Q2 are 'off' (more drive) they allow the 100K reistors to effectively remove the bypass caps (C8 & C9) from the circuit.
Hense, more gain.
My feeling is that you have an existing oscillation & the bypass caps are masking it.
I would measure the Vac ripple on the power supply nodes, as a start.
You may have a duff cap.

[CornElvis]

So the 'more drive' when selected makes an F# tone by itself?
With or without a guitar plugged into the input?
There is not all that much to the 'more drive' circuit.
U3-A (TP36) provides +16Vdc to the gates of Q1 & Q2 (JFet J111).
J111 is an N channel jfet, meaning they are always 'on' & need a gate signal to turn them 'off'.
The 'off' signal is provided by U3-A.
When Q1 & Q2 are 'off' (more drive) they allow the 100K reistors to effectively remove the bypass caps (C8 & C9) from the circuit.
Hense, more gain.
My feeling is that you have an existing oscillation & the bypass caps are masking it.
I would measure the Vac ripple on the power supply nodes, as a start.
You may have a duff cap.

It doesn't matter if the guitar is plugged in or not. the switching is OK, so in my opinion the JFets are OK. The thing is this thing had a short circuit before I touched it, it might be that more components are malfunctioning without telling me straight away. All the checkpoints are OK or they are within the 10% range

[soundguruman]

I think those "illinois capacitors"
puff smoke, and well, I didn't know Illinois was in China.

[Jazz P Bass]

Start by pulling the peamp tubes one at a time.
This may help narrow it down.
You have to know if it is in the preamp, power amp or if it is global.

[CornElvis]

It's the preamp, if I plug something in the power amp in the problem is gone and it only happens with the second channel "more drive" button

[soundguruman]

It's the preamp, if I plug something in the power amp in the problem is gone and it only happens with the second channel "more drive" button

socket the fets and op amps. Then they can be pulled and replaced more easilly...

It's a total pain to pull the board, but when you do, put in sockets.
If you do not have the skill to unsolder parts and install sockets, get a tech to help.
The trouble with these boards is that they are really easily damaged from soldering.

[Enzo]

the switching is OK, so in my opinion the JFets are OK.

Not sure I agree with this. The switching turns on your oscillation, that is hardly OK.. The JFETs may not have anything to do with it, but you cannot rule them out with that thinking. The amp "switches" by way of relays by way of op amps and bipolars if you look at the LEDs, but the JFETs do the actual gain control. If one JFET has a leak to its gate, and those gates are indeed wired together, then potentially a feedeback path can result. The fact that the circuit seems to switch between two channels doesn't mean all elements of that switchover are working right

I doubt it is the problem, but substitute a different 12AX7 into V2. The whole drive channel is noting more than adding in V2b. And the More Drive is simply those two JFETs controlling cathode bypass caps in the two places. The two cathodes are also test points. TP9, TP12 both show a nominal 1.93VDC. I am not concerned if it is off a little in voltage, but what I don;t want to see is some substantial change when MORE DRIVE is pushed. Easy to monitor and check.

With Drive on, but More not enabled, use a clip to short across R23 and R24. One at a time. Does either one trigger oscillation? Basically one end of those is already grounded, we want to ground the hot end, enabling the cap. That is all the JFET does. This clip lead method lets you control each cap without turning on the More circuit.

ANother possibility is in fact the filter caps, try clipping an extra cap in parallel with C36. C36 is the filter cap for the "X" B+ node, which this circuit uses. If it is weak the lack of good decoupling could add to instability.

Jazz, caps IN circuit is the higher gain cocndition. Out is lower gain. Bypass caps increase the gain.

I might be missing something, but I don;t see how problems at the power tubes - such as shoving them in the wrong way and burning those resistors - could affect this preamp circuit.

Sockets for transistors? Oh please.

[soundguruman]

Not sure I agree with this. The switching turns on your oscillation, that is hardly OK.. The JFETs may not have anything to do with it, but you cannot rule them out with that thinking. The amp "switches" by way of relays by way of op amps and bipolars if you look at the LEDs, but the JFETs do the actual gain control. If one JFET has a leak to its gate, and those gates are indeed wired together, then potentially a feedeback path can result. The fact that the circuit seems to switch between two channels doesn't mean all elements of that switchover are working right

I doubt it is the problem, but substitute a different 12AX7 into V2. The whole drive channel is noting more than adding in V2b. And the More Drive is simply those two JFETs controlling cathode bypass caps in the two places. The two cathodes are also test points. TP9, TP12 both show a nominal 1.93VDC. I am not concerned if it is off a little in voltage, but what I don;t want to see is some substantial change when MORE DRIVE is pushed. Easy to monitor and check.

With Drive on, but More not enabled, use a clip to short across R23 and R24. One at a time. Does either one trigger oscillation? Basically one end of those is already grounded, we want to ground the hot end, enabling the cap. That is all the JFET does. This clip lead method lets you control each cap without turning on the More circuit.

ANother possibility is in fact the filter caps, try clipping an extra cap in parallel with C36. C36 is the filter cap for the "X" B+ node, which this circuit uses. If it is weak the lack of good decoupling could add to instability.

Jazz, caps IN circuit is the higher gain cocndition. Out is lower gain. Bypass caps increase the gain.

I might be missing something, but I don;t see how problems at the power tubes - such as shoving them in the wrong way and burning those resistors - could affect this preamp circuit.

Sockets for transistors? Oh please.

Fender Hot Rod Deluxe.pdf
I think Q1 or Q2 is bad. Installing a socket is intelligent, cause you can change it without pulling the board.
Saving quite a bit of time and money in future failures.
Also, it prevents the transistor from being heat damaged from the soldering.
But since Enzo hates sockets, (oh please) next time, have him SOLDER your tubes in.


The factory should have installed sockets to begin with.

It could really be anything in the foot switch circuit, that circuit is the hamster wheel.

Tektronix put FET sockets on their boards across forty years. But we all know how stupid they are. FETs never go bad.

[Enzo]

You know, if you were not such an ignorant moron confusing people, you;d just be some jerk.

I have nothing against tube sockets, what I do have something against is someone suggesting we put sockets under a bunch of parts so we can change them out easier. That is sophomoric at best. Removing more parts than necessary and soldering in sockets for them is adding just that many more connections to fail.

Saves time in future failures? How many times do we really think those two JFETs are going to fail?

And as to damaging the parts by soldering, the trick is to learn how to solder properly. Those little TO92 transistors are not going to be hurt by soldering at their leads unless the tech is just a solder goon.

[soundguruman]

I think that the best way to deal with this footswitch circuit is to take the board out once and put sockets in.

The op amp goes bad, the FETs go bad, and people are having problems with that quite often.

This circuit board will not survive repeated soldering. It falls apart. The sockets preserve the board when it needs repairs.
Many of the best transistor and solid state, digital circuits use sockets. It makes servicing faster, easier and protects the parts and board from damage.

Incidentally, FETs go bad frequently. They are easily damaged by soldering, especially when an inexperienced person is installing it.

[Enzo]

The same inexperienced person you want to tear out a bunch of parts and install a bunch of sockets. Each IC socket they install will be 8 more chances to destroy a copper trace or solder pad.

Yes, on the internet we can find numbers of people having problems with this circuit, but there are tens of thousands of these amps, they have been making them for YEARS. The percentage of units with these problems is very LOW.

As to many of them using sockets, show me. Once in a while a particular part will be socketed for some special reason, but all the transistors in an amp? Where might we see one like that? Peavey used to socket ICs in the 1980s, they stopped. Too many intermittancies. And let us limit this to guitar amps. I don;t care what they did in your oscilloscope, it doesn't have to bounce around the trunk of your car and sit on top of a vibrating speaker cab each night.

[Jazz P Bass]

Jazz, caps IN circuit is the higher gain cocndition. Out is lower gain. Bypass caps increase the gain.

Yup, got that one bass ackwards.
Thanks Enzo.

[soundguruman]

[QUOTE=Enzo;269099]You know, if you were not such an ignorant moron confusing people, you;d just be some jerk.

You are entitled to your personal opinion, and entitled to participate in any discussion.
BUT what you are not entitled to is personal attacks, insults and name calling.
That is a violation of forum rules.

[J M Fahey]

Funny you piss off everybody *so much* that name calling is the least that can be done, and then you fake surprise and "don't know why everybody hates me".
Well, take responsibility for your acts.