Looking for help with A old groove tube STG -

[ffarrell]

I came across a 1984 GT STP G unit that had sit Iin a warehouse for 15 years untouched.

There was no power tube and after opening the hood I found a burnt 5 watt res.at the power filter caps. 

So I found the 5 watt res and replaced it and also noticed the caps were all loose to their pads  so I hit all the solder joints i could find. 

Powered it up and it worked but not the best sounding unit.

So knowing caps go bad over time and wanted to make this a Boutique ish. I replaced all the filter caps with F/T caps. 

Sounds better but still want it better.

Replaced V1 with a 12at7 still closer.

Also have tried a set of el34 for testing.

The problem is....
The amp seams to squeeze down to much and the low is not clean.

Plate voltage is 425.

Any ideas where to look next?

Thanks
fvf

[Willabe]

Can you post a schematic?



            Brad     

[ffarrell]

Hi

Aspen sent what he had as a jpeg.

here it is

thanks
fvf

[HotBluePlates]

... Sounds better but still want it better. ...

I don't know what "better" is... You have a sound now, so what needs to change?

The problem is....
The amp seams to squeeze down to much and the low is not clean.

"Squeeze down"? Does that mean the amp compresses too much?

I don't see anything in the drawing that indicates compression. I do see a +/-28vdc supply, but no solid-state stuff is shown, telling me the schematic is incomplete. I can't know if there's some sort of compression circuitry there.

I see 4 gain stages before the phase inverter when the boost stage is engaged. You can get a pretty severe level of distortion with 4 gain stages, though that is tempered somewhat by the largish voltage divider leading into the 4th stage of 220kΩ+3.3MΩ / 470kΩ.

BTW, there's likely a mistake in the drawing in the phase inverter: the 25kΩ Presence pot likely runs from the 6.8kΩ resistor to ground, with the 0.1uF cap in parallel (not in series, as drawn). As drawn, there's not a d.c. path to ground for the phase inverter except through the output transformer, and that would probably make the feedback loop not function as intended.

"... Low is not clean." Does this mean the bass distorts before the treble? Or that you get farty bass under distortion? Or??

If there is distortion in the bass when the amp is "playing clean" then there may be a faulty connection or an issue with the output transformer. If the problem is while the amp is distorting, it may indicate too much bass, as all gain stages have 25uF cathode bypass caps. Normally, high-gain amps use smaller-value bypass caps to reduce bass. They do this to prevent the bass notes distorting (or distorting so much), which then have harmonics which beat against midrange fundamental tones.

... I found a burnt 5 watt res.at the power filter caps.  ...

The 1kΩ 5w resistor is in the power supply after where the output tubes tap power. So the phase inverter or preamp had to be drawing too much current. Is there really no connection/wire from the junction of the 1kΩ and 5.6kΩ resistors to something in the amp? It looks like this could/should go to the output tube screens.

Knowing if that is an error/omission in the schematic would influence my guess about what caused the burned resistor.

[ffarrell]

Hi Hot Blue Plates

Thank you for taking a look at this for me.

"I don't know what "better" is... You have a sound now, so what needs to change?"

I'm just so new at the tweaking of a amp that I am a bit helpless.

At this point the amp sounds a bit compressed over all if one lays into it like a open A chord or B bar chord it pulls down and like a 1.5 to 1 ratio and releases at maybe 250 ms.

also the bass notes like if one was to play Tom Petty "Running down a dream" lick the bass is a bit distorted.

"... Low is not clean." Does this mean the bass distorts before the treble? Or that you get farty bass under distortion?"

The low end was farty but going to the 12at7 made that less apparent.   The amp is working but is a bit crunchy sounding also. 
 If I were to set my preamp knob on 2 or 3 and my tone knobs all on 4 and my master volume on 7 or 8 I would expect the ampto not have any overdrive or ddistortion but mine has a bit.

Also the bass knob seam to work mostly in the first 1/3 of its turn.

Could this be a bias problem?



My goal is to have a amp with rich overtones in the blues realm somewhat like Dickie Betts - Derek Trucks ish.


" I do see a +/-28vdc supply, but no solid-state stuff is shown, telling me the schematic is incomplete. I can't know if there's some sort of compression circuitry there."

Good Eyes,  It is for the part they left out of the jpeg,  a cabinet sim circuit, but with it's own direct output.  There is a 5532 and a few more bits but I don't have any paper on it and I don't think it's part of the power tube drive.

Please let me know how I can help you , help me.

thanks
fvf

[HotBluePlates]

... At this point the amp sounds a bit compressed over all if one lays into it like a open A chord or B bar chord it pulls down and like a 1.5 to 1 ratio and releases at maybe 250 ms.

the bass notes like if one was to play Tom Petty "Running down a dream" lick the bass is a bit distorted. ...

But "Running Down a Dream" is something like a wide-open Deluxe Reverb, set for a really bright sound. Meaning it's a distorted sound. The compression thing may be an issue.

For testing, you should have the Master at max volume, the Gain switch (above the input jack) off, and probably the input pad pull pot pulled out. The pictures I've seen show both a "Gain" and "Preamp" control, but it's not very obvious in the schematic where each is in the circuit. I'd guess that one of them being turned to zero will kill all volume when the Gain switch is engaged (because it only feeds the triode connected by this switch), while the other will kill all volume regardless (because it's in the circuit all the time).

So figure out which is which, and keep the extra stage's volume off. Now you should be getting some distortion if you turn the remaining preamp volume control about half-way. That would probably be normal. Of course if you kick in the exta gain stage, you'll probably have a hard time getting a clean sound at any volume.

If the amp compresses under distortion, that's normal, because that's part of what distortion is/does. If the amp compresses on clean sounds, that may be a problem.

If you're using any of the speaker emulator or line out circuit during all this, all bets are off.

My goal is to have a amp with rich overtones in the blues realm somewhat like Dickie Betts - Derek Trucks ish.

If you think about it, those players use a TON of distortion. Even more, when you record and are playing single-note lines, the amp may sound cleaner than it really is in the room. That is, you'll notice the distortion jump out when you play double-stops.

You might try recording yourself playing through the amp to see if your impression of the amp changes. Also, both those players made liberal use of their guitars' volumes controls to adjust clean/distortion...

[ffarrell]

... At this point the amp sounds a bit compressed over all if one lays into it like a open A chord or B bar chord it pulls down and like a 1.5 to 1 ratio and releases at maybe 250 ms.

the bass notes like if one was to play Tom Petty "Running down a dream" lick the bass is a bit distorted. ...

But "Running Down a Dream" is something like a wide-open Deluxe Reverb, set for a really bright sound. Meaning it's a distorted sound. The compression thing may be an issue.

For testing, you should have the Master at max volume, the Gain switch (above the input jack) off, and probably the input pad pull pot pulled out. The pictures I've seen show both a "Gain" and "Preamp" control, but it's not very obvious in the schematic where each is in the circuit. I'd guess that one of them being turned to zero will kill all volume when the Gain switch is engaged (because it only feeds the triode connected by this switch), while the other will kill all volume regardless (because it's in the circuit all the time).

So figure out which is which, and keep the extra stage's volume off. Now you should be getting some distortion if you turn the remaining preamp volume control about half-way. That would probably be normal. Of course if you kick in the exta gain stage, you'll probably have a hard time getting a clean sound at any volume.

If the amp compresses under distortion, that's normal, because that's part of what distortion is/does. If the amp compresses on clean sounds, that may be a problem.

If you're using any of the speaker emulator or line out circuit during all this, all bets are off.

Hi and thanks again for looking at this.  The amp is set up in a way that when the switch is set to Gain the Pre pot is bypassed. and vice versa.  The bass, mids, and highs pots kill the amp ( no sound comes out ) if all the tones are turned off.  A normal set up for me is master vol around 8 out of 10 and Pre up until it I start to hear some distortion.  ( around 3 out of 10 )

I have the highs and mids around 5 of 10 and bass around 4 of 10.  There is a substantial boost with pulling the mids pot and also with pulling the treble pot. 

My goal is to have a amp with rich overtones in the blues realm somewhat like Dickie Betts - Derek Trucks ish.

If you think about it, those players use a TON of distortion. Even more, when you record and are playing single-note lines, the amp may sound cleaner than it really is in the room. That is, you'll notice the distortion jump out when you play double-stops.

You might try recording yourself playing through the amp to see if your impression of the amp changes. Also, both those players made liberal use of their guitars' volumes controls to adjust clean/distortion...
[/quote]

I'm out on tour right now but I get home in a week and would like to post some audio tracks just so you can hear what I hear, I hope. 

thanks again
fvf

[HotBluePlates]

The bass, mids, and highs pots kill the amp ( no sound comes out ) if all the tones are turned off. 

That's 100% normal for the type of tone circuit this amp uses. The same thing happens on blackface Fender amps. Just don't turn all tone controls to zero. 

I'm guessing that at the end of the day you're thinking this amp doesn't do clean, loud enough for you. I'll see if the clips say something different, but maybe the amp just isn't as powerful as you'd like it. Or maybe it does a flavor of distortion you don't like. I dunno...

[ffarrell]

Hi:

Ok, here is a link to the sound files on my dropbox. 

https://www.dropbox.com/sh/jnczwxndbhpmy2g/encAP5IgP5


I've attached to pic to see the settings of the amp.

thanks
fvf

The bass, mids, and highs pots kill the amp ( no sound comes out ) if all the tones are turned off. 

That's 100% normal for the type of tone circuit this amp uses. The same thing happens on blackface Fender amps. Just don't turn all tone controls to zero. 

I'm guessing that at the end of the day you're thinking this amp doesn't do clean, loud enough for you. I'll see if the clips say something different, but maybe the amp just isn't as powerful as you'd like it. Or maybe it does a flavor of distortion you don't like. I dunno...

[HotBluePlates]

I listened to the sound clips, and I think I'm hearing "normal operation" from this amp.

I say that because the preamp is 3 gain stages (like the High input of a JCM800) with the Gain switch off, and 4 gain stages (like a Soldano) with the Gain switch on. A typical clean channel in a multi-channel amp would use 2 gain stages, like a blackface Fender.

If I had some voltage measurements from you amp, I'd could do a more thorough assessment of the sensitivity at the input jack (in other words, how big an input signal at the input drives the amp to distortion). However, I'm thinking you're not gonna get much clean out of this without changing what it is (like possibly getting rid of cathode bypass caps in the preamp and/or losing a gain stage).

Maybe let this be high distortion and augment it with a clean-only amp?

[ffarrell]

I would love to get the voltage readings you need if you tell me where.  I don't mind changing
this amp into something a little more pleasing to me.  Dose it sound like the bias is off. 

What you heard is with matched el34 and a 12at7 in v1.

it's the closest to the sound I would like.to try and get out of the thing.

The 6v6 and 12ax7 were breaking up to quick.

I plan to build a clone amp later, maybe a AC30 ish or a Marshall 50 watt.

Once again thank you for your time on this.
fvf

I listened to the sound clips, and I think I'm hearing "normal operation" from this amp.

I say that because the preamp is 3 gain stages (like the High input of a JCM800) with the Gain switch off, and 4 gain stages (like a Soldano) with the Gain switch on. A typical clean channel in a multi-channel amp would use 2 gain stages, like a blackface Fender.

If I had some voltage measurements from you amp, I'd could do a more thorough assessment of the sensitivity at the input jack (in other words, how big an input signal at the input drives the amp to distortion). However, I'm thinking you're not gonna get much clean out of this without changing what it is (like possibly getting rid of cathode bypass caps in the preamp and/or losing a gain stage).

Maybe let this be high distortion and augment it with a clean-only amp?

[HotBluePlates]

I would love to get the voltage readings you need if you tell me where. 

Pin 5 of the output tubes (should be a negative voltage), and pins 1, 3, 6 & 8 of all the preamp tubes. Also points A, B, C, D of the power supply, as shown in the schematic image you posted.

That's probably more than I need to figure it out, but might as well be thorough.

[ffarrell]

Pin 5 of the output tubes 

-31.75  _ -31.80



          1      3         6       8       of all the preamp tubes.

V1   149   2.21     154   2.11
V2   213   1.45     NC      NC
V3   203   1.76     349   62.7
V4   252   26.90   240   26.90 *   3 and 8 are bussed


A,      B,      C,     D      of the power supply:
406  349   408   428

readings taken from the + of the filter caps.

thanks
fvf

I would love to get the voltage readings you need if you tell me where. 

Pin 5 of the output tubes (should be a negative voltage), and pins 1, 3, 6 & 8 of all the preamp tubes. Also points A, B, C, D of the power supply, as shown in the schematic image you posted.

That's probably more than I need to figure it out, but might as well be thorough.

[HotBluePlates]

I forgot to tell you to measure d.c. volts on both sides of the 680Ω phase inverter resistor, but no matter. We'll move forward without it.

The output tubes are biased to ~-32v and therefore need a 32v peak input signal (to a single tube) to drive them to full clean output power.

We can probably safely estimate a gain of 30 from the input of the phase inverter to either output. A gain of ~60 would be typical for a 12AX7 stage, and the long-tail is generating 2 outputs with a single input voltage, so gain from the input to either output alone is about half what it normally might be. 32v / 30 = ~1v peak input to the phase inverter to generate full clean output power.

We'll assume the 1MΩ master volume between V3A and the phase inverter is at maximum.

V3A has a 100kΩ plate load, but if the master is full-up, it also sees two parallel 1MΩ pots as a load. Therefore the dynamic load is 100kΩ ll 500kΩ = ~83kΩ. Cheating a little to reduce estimation, the gain of the stage is found by the intersection of the green 83kΩ loadline with the gridlines. Gain = (275v-110v)/(3v-0v) = 55. Therefore, 1v peak/55 = 18.2mV peak input to this stage gives full clean output power.

There is a voltage divider leading into V3A made up of 220kΩ, 3.3MΩ & 470kΩ, with V3A's grid at the 470kΩ. That gives a "gain" (really attenuation) of 470kΩ/(220kΩ+3.3MΩ+470kΩ) = 0.118. So we need 18.2mV peak / 0.118 = 154mV peak at the input of the voltage divider for full output power.

Your Gain switch is off, so V2A is bypassed.

I assumed V1B has a supply voltage (Node A) of ~300v, as that was implied by the tube current, plate voltage and plate load resistor. I also assumed you have a 12AT7 in the socket. The voltage divider at V3A's grid is large enough that there's little error in assuming the dynamic load for V1B is 100kΩ. Gain = (200v-34v)/(4v-0v) = 41.5. Therefore, 154mv peak/41.5 = 3.71mV peak input to this stage gives full clean output power.

Ironically, because the 12AT7 is also used counter to its nature in low-current, high impedance circuitry, it distorts more than the 12AX7 in V3A. In other words, it gives less amplification, but its characteristic curves are so bent down where it's running that it has about double the distortion of a 12AX7 in that spot (assuming small signals).

The Preamp Volume control is not indicated anywhere on the schematic. I'm will assume that it follows the tone stack, like a blackface Fender amp (which stage 1&2 seem to copy anyway). You have your Preamp volume set around 2-3. A true 10% taper log pot set to 3 would only have ~4.8% of its total resistance to ground, which gives a "gain" (loss) of 0.048. 3.71mV peak / 0.048 = 77.3mV peak.

V1A has an internal plate resistance at the operating point of ~23kΩ. 23kΩ ll 100kΩ = 18.7kΩ source impedance for the tone stack (used to tweak the Duncan Tone Stack Calculator model). TSC shows a loss of 11-12dB for lows & highs at the settings in your pictures, and a midrange loss of 22dB.

-12dB converted to a voltage ratio equals  -> -12dB / 20 = -0.6  ->  10^-0.6 = 0.251, or about 1/4th
-22dB -> a voltage ratio of 0.079, or ~1/12.6

Accounting for the loss of the tone circuit, 77.3mV peak / 0.251 = 308mV peak at Bass/Treble or 77.3mV peak / 0.079 = 978mV at 500Hz at the input of the tone circuit will make full clean output power.

V1A is essentially the same as V1B, so we'll assume the same gain of 41.5. So an output of your pickup of 308mV peak / 41.5 = 7.42mV for Treble/Bass gives full clean output power, or 978mV / 41.5 = 23.6mV for 500Hz.

The average pickup, whacked hard should make 100mV pretty easily, and turning your guitar volume to half still gives 10mV, so you see even these settings cause the output tubes to distort. All preamp tubes have distorted some as well (I found 2nd harmonic distortion between 2-10% for various stages) even before they're hit hard enough to either cutoff or saturate.

From what I've found, your distortion issue is "normal" for the circuit in the STP-G. In fact, I'm almost certain distortion was the whole point.

[ffarrell]

Wow great reply.  I must reread thus a few times to try to understand it.  I think your correct.  It is what it is.  

One thing that still puzzles me.  In high gain mode all the EQ works as I would expect but in standard mode the bass only works to 4 or 5 and little to no change after that.  

Also can you tell me if there is a easy way to reduce the Gain in the Gain mode so it starts a bit less like as if it would start at -3 and go to 7

thanks again

fvf

[HotBluePlates]

Also can you tell me if there is a easy way to reduce the Gain in the Gain mode so it starts a bit less ...

You mean, once you're in the high gain mode how do you get it to have a little less gain? Or make the Gain knob act as though it's turned down a bit?

If yes, the easiest way is to add a 1-3MΩ resistor between the Gain switch and Gain pot. That will make it like it can't be turned all the way up (to what it can do now). The exact value depends on your experimentation and hearing what you're looking for.

... the bass only works to 4 or 5 and little to no change after that.   

The Tone Stack Calculator shows a 17dB change in bass level when you dial the Bass control from 0 to 5. From 5 to 10 there is only a ~5dB change in bass level. So it's the nature of the circuit.