Problem with a Tube guitar preamp project .....

[Minion]

Hi folks ... I am working on a Simple tube guitar preamp project and I am haveing problems ....

here is the schematic ...

The EQ section works fine but the tube preamp section is giveing me headaches ...

Here is the Problem ...

I turn on the power switch and after a couple seconds (as the heaters warm up ??) I start to hear and nice guitar tone but it quickly breaks up into a squeeling feeding back mess....

The heaters are at 12.8v AC and the Plate is at about 70v DC , there is a Voltage doubler comeing off of the 12.8v AC to make a +/-15v supply for the Opamp which is a OPA2107 , the Voltage doubler has 2 100uF caps per rail with a Voltage divider or two 33k resistors going from each rail to ground ......

The squeeling happens with the Pot between the Tubes all the way down and with the Input opamp gain all the way down .....

I"m thinking that maybe the tube Bias is off or something but this is only my second foray into the tube world and am severely lacking in tube knowlege ??

anyone can help me here ??

Thanx a Lot ...

 

[okgb]

How is your wiring ? neat or messy
inputs crossing outputs in a high imp world ?
try shielding inputs ,  decouple the power supplies
wiggle the wires if you haven't already
post a pic of the actual unit .
Is it your own design or from a site ?

 

[Minion]

All Inputs and outputs are useing shielded wire (Foil and Braid) as well as the pots ......

The design is based on a PaIa kit , the problem may be that the Original design ran off of a 35v Plate voltage and I didn"t adjust the Cathode or plate resistors for the Higher Voltage as I wouldn"t know how to go about it ....


Thanx

 

[rodabod]

Does it squeal with the input opamp disconnected? If not, maybe try adding a feedback cap in parallel with the 250K pot on the first opamp.

 

[okgb]

wonder whether  a small cap across the plate load or from plate
to grid would help
but you're right find the cause before you fix it
I don't think a mis bias would cause squeal , just change
the cut off and operating point but maybe a smarter tube guru
can tell how to properly bias [ or google it ]

 

[Butterylicious]

Ain't gonna work like you got it.  I can't even get it to flow any current.

When all else fails, steal from Leo.

150V B+, 100K plate resistor, 1K5 cathode.  Works every time.  1st stage will have a gain of about 26 and the 2nd a gain of about 29.  Cathode bypass caps push either stage up to a gain of 59.

-Richard

 

[okgb]

Hey cool looking program

if you can find a 1:1 xfmr for the A.C. , that should get enough
for 150 v  , still many run on less , the rolls is 90ish v i believe

 

[Butterylicious]

The low B+ is what's making this difficult.  Minion's original part values work fine at 290V.

The only way I can get a 12AX7 to work at 70V is to drop the plate resistor to well below Rp of the tube which is a nono.

>Hey cool looking program
John Broskie's TubeCad is way cool.  $39.00 can't beat that with a stick.

-Richard

 

[rodabod]

At first glance, I don't really see why it wouldn't work.

 

[peterc]

I have had this before when building preamp stages with a lot of gain.

Try a 100pf cap across the 2nd plate resistor, if no go, the try it across the first plate resistor.

If it works in one of these places, it will probably roll off a lot of high  end. Try reducing the size until it stops the squealing but doesnt roll off too much high end.

Peter

 

[Curtis]

At first glance, I don't really see why it wouldn't work.

Me either.  LTSpice reckons it works fine at 70V with the values given, about 28dB gain per stage.

Regarding the squealing try swapping in a different valve in case your current one is microphonic.  Layout will be important - keep plate outputs away from grid inputs.

Try shorting each stage grid to ground in turn and see if the squealing stops.  Should help you isolate which stage is causing you grief.

 

[Butterylicious]

I'm totally curious what parameters LTSpice is returning, mainly Vtube, plate current, and output symmetry.

 

[Curtis]

LTSpice says:

Vtube (I assume this means cathode-to-plate voltage) = 25.8V
Plate current = 156uA

Feeding a 1.8VRMS sine at the grid, and probing the plate shows the tops being clipped at 48V and the bottoms rounding off at 3.8V, so about a +/- 22V swing.

Real life may differ of course...

 

[abbey road d enfer]

Hi folks ... I am working on a Simple tube guitar preamp project and I am haveing problems ....

here is the schematic ...

The EQ section works fine but the tube preamp section is giveing me headaches ...

Here is the Problem ...

I turn on the power switch and after a couple seconds (as the heaters warm up ??) I start to hear and nice guitar tone but it quickly breaks up into a squeeling feeding back mess....

The heaters are at 12.8v AC and the Plate is at about 70v DC , there is a Voltage doubler comeing off of the 12.8v AC to make a +/-15v supply for the Opamp which is a OPA2107 , the Voltage doubler has 2 100uF caps per rail with a Voltage divider or two 33k resistors going from each rail to ground ......

The squeeling happens with the Pot between the Tubes all the way down and with the Input opamp gain all the way down .....

I"m thinking that maybe the tube Bias is off or something but this is only my second foray into the tube world and am severely lacking in tube knowlege ??

anyone can help me here ??

Thanx a Lot ...

Just looking at the schematics, it tells me there's way too much gain. It probably has the right gain when operating in starved mode (B+=40v), but with full anode voltage the gain is probably at least ten times more than expected. Even with the pot down, there is still ca. 45dB voltage gain around the tube section. No wonder you experience some feedback.

 

[Minion]

How would I configure the tube stage to have less gain ??

I haven"t had a chance to continue debugging yet but I have a feeling that maybe the opamp is oscilateing because I have gotten simular sounds from opamp guitar pedal curcuits when oscilateing badly... I"ll try bypassing the Opamps and see what happens ....

Then if that doesn"t pan out I"ll try a different voltage on the Plate , I have a +35v DC rail available that has 20,000uF of filtering ......

Thanx for the help guys ...

 

[abbey road d enfer]

How would I configure the tube stage to have less gain ??

I haven"t had a chance to continue debugging yet but I have a feeling that maybe the opamp is oscilateing because I have gotten simular sounds from opamp guitar pedal curcuits when oscilateing badly... I"ll try bypassing the Opamps and see what happens ....

Then if that doesn"t pan out I"ll try a different voltage on the Plate , I have a +35v DC rail available that has 20,000uF of filtering ......

Thanx for the help guys ...

First reduce drastically the resistor at the foot of the pot between the two triode sections; you can even short it. That will allow you to run the second triode with a normal level.

 

[Minion]

OK, Here is an Update , I took out the opamp and bypassed it so it was just the tube , and the Feedback and squeeling went away and get a nice guitar tone accept there is this wierd sound in the backround , sort of like slow motorboating but sounds a bit like a star wars Lazer gun ....

Do you guys know what might cause this ??

Thanx

PS : I tried it with the opamp back in and a 680pf cap in the feedback loop and put a .0047cap from + to - with no change , I"m getting a steady 16.5v on each rail of the opamp ....

 

[Curtis]

Try adding 100nF decoupling caps between the positive and negative supplies to ground as close as possible to the opamp power pins.

Got a diagram of your power supply? 

 

[rodabod]

there is this wierd sound in the backround , sort of like slow motorboating but sounds a bit like a star wars Lazer gun ....

Do you guys know what might cause this ??

Sounds like oscillation to me. I'd maybe try lowering gain somewhat... Try dropping the first anode resistor to ~120K and see f this helps. Ideally, you should work with the tube's curves to choose new values for your chosen supply voltage. In order to get this thing sounding good, you need to get the right amount of gain / clipping at each stage, and possibly eq'ing too.

I don't see anything wrong with band-limiting from say 100Khz down to 20KHz or whatever if this unit is quite high-gain, as long as the unit still sounds fine for guitar.

 

[Minion]

Thanx rodabod , I"ll try that ... I"m going to try a couple other things also , I have +/-12v Regulated available that I am running the EQ board with , so I"m going to try tapping those Voltages to run the Opamp and see if that stops the oscilateing of the opamps , I have a feeling the opamps aren"t likeing the Voltage doubler PSU , I might also try running the heaters off of the +12v DC Regulated supply and see if that helps with the Hum .....

I"ll post back and let you know how it goes ....

Thanx