Help Restoring and Tweaking Gretsch Tube Spring Reverb - G6144

[whoops]

Hi,
I got a Gretsch G6144 Spring Reverb sometime ago. When I received it it was in really bad shape, power chord was disintegrating, and had a lot of hum.
I restored the Cosmetics, installed a new 3 prong power chord and replaced all the Electrolytic caps.






The unit works, but there's still some hum, it's really dark sounding quite lo-fi.
I'm a mixing Engineer so I want to use it as an FX during mixdown most of the time and sometimes I might use when I play guitar.
I used to use a Fender Spring Reverb for mixdown and worked very well for my needs, I know both circuits are quite different and that the Gretsch will never be the Fender but I would like to see what could be improved and learn more while doing it.

The Hum and Lo-Fi sound is present with any of the following setups:
- Guitar in - Out to amplifier
- using in mixdown with reamp box and DI at the output
- using in mixdown without reamp just attenuating the soundcard Output, and output from Greatch to soundcard DI in



The stock reverb tank is a small cardbox tank, I tried to replace it for an OC electronics tank I had around and the sound improved dramatically, more High end and reverb effect. Sounded actually like a spring reverb and less like a Lo-Fi Gizmo.
So I will replace the stock tank for sure.

But for now as a learning experience on restoration and tweaking I would like to see what could be improved or tweaked circuit wise.

[whoops]

I analyzed the schematic and compared it to the circuit.
Found some errors.

Also did a plan of possible improvements to the circuit, and would like to ask your help and opinions.

My goals are:
- improve the PSU scheme and filtering were possible
- reduce the amount of hum were possible
- improve the High End response as I'm using it for music mixing. Using it mostly for attenuated unbalanced Line level signals.
(stock unit is much more dark and lo-fi than a Fender Spring Reverb unit)


here is the schematic with my notes and ideas:

[whoops]

(EDIT - I will keep updating this post with the decisions based on help and info of fellow members)

would like to ask for your help and guidance in the following Notes

NOTE 1
470K for the grid stopper resistors seems a really high value and also a possible source of Hum.
I was thinking in reducing this value enough so that it is still efficient at RF frequencies but less susceptible to PSU induced hum.

I only need 1 input to be honest so I could use 33K has a grid stopper value.
But I was thinking that for my uses maybe having the 2x68K Fender type input would be nive, since I had a High and Low input, could use the High input for when I use the unit with guitar and the Low input when I used it in mixdown with unbalanced line level singals.
What do you think?

On the schematic the Grid stopper is before the grid leak resistor, should I move the grid stopper to after the grid leak resistor and solder it directly to pin7?

As per PRR and 2deaf advice, I will change the grid stopper resistor for 33k. Will be soldered directly at the tube socket Pin and after the grid leak resistor. Input jack will be changed so it shorts the input when no jack inserted.


NOTE2
The grid leak resistor is 2,2megs, this also sets the input impedance.  I see most guitar tube amp inputs use 1Mega as the gread leak resistor.

Practically will make a difference if I use 2.2M or 1M?
Do you recommend changing the Grid Leak to 1M?

Grid Leak resistor will be changed to 1Mega

NOTE 3
Some cap and resistor values are different between the schematic and the circuit.
Coupling Cap after 6DR7 first stage is actually 0.02u on the circuit and not 0.05u like in the schematic.
I guess 0.02u will roll off more bass going to the next stage.

What do you thing is better here schematic value or circuit value?

This cap will stay at 0.02uf

NOTE 4
I'm guessing this is a filter cap.
Wouldn't the 0.05u schematic value be better here than the 0.02u found in the circuit?

22uf/450v Cap will be used here

NOTE 5
there's 2x270K resistors that actually measure 304K and 306K, these are old carbon composition resistors. They're in the 15% and 20% tolerance range but I was thinking as a restoring procedure to replace all resistors that measure higher than 10% of tolerance.

NOTE 6
There's another 100K resistor in the circuit parallel to the one drawn in the schematic.
I don't know if it was a factory assembly error, but it might be.
The 2 paralleled 100K resistors form a 50K resistor.
What's your opinion, should I use 100K or 50K here?

Will use 50K.
The use of 50K was probably a later revision not shown in the schematic. The Airline reverb schematic confirms this.

NOTE 7
Filter Cap
Would you recommend 0.1u or 0.05u here? Maybe increase this value

22uf/450v Cap will be used here

NOTE 8
The second section of the 6EU7 (output) is arranged as a cathode Follower
For the DC blocking/Coupling cap Schematic says 0.05u but circuit has 0.02u, I don't know if more bass roll off is needed at the output, I'm tempted to increase the 0.02u value.
What do you think?

Will Keep the 0.02u value

NOTE 9
In place of a 100K resistor the circuit has 1.1M resistor fitted , thats a big difference.
Is this setting the Output impedance?
If so isn't 100K a better value to use?

1.1M resistor will be kept

NOTE 10
The filtering arrangement was different than the schematic when I got the unit, and I don't remember how it was originally as it was a long time ago. At that time I didn't notice there was no dot between at this connection, so I rewired the circuit like the dot was there.
Is the missing dot in the schematic a mistake like I assumed?

It's a small mistake in the Schematic, there should be a dot there.

NOTE 11
The Maximum value for the first filter cap in an 5Y3GT is 32uf, the circuit uses 20uf. Should I increase the capacitor from 20 to 32uf?

Filtering capacitor values will stay the same. This caps were already replaced, with 22uf instead of 20uf.

NOTE12
Heaters will be rewired and an artificial center tap will be installed.

NOTE13
I will remove the Reverse switch on the Mains. This switch in conjunction with the 0.02 cap form the Death Cap found in old Fender amps.
I don't need or want a Reverse mains switch and the 0.02 filter cap.
I will rewire the 3 prong mains cable exactly like the recommendations to convert 2 prong fender amps to 3 prong.
High to on/off switch, then to Fuse , then to Power transformer primary A
Neutral directly to Power transformer primary B


Thank you so much for following this learning experience and for the help you can provide

Regards

Tiago

[PRR]

It would help to know if the hum and hi-cut are in the direct path, the reverb, or both. There are two stages in common and 2 not.

It clearly was "too bright" once. The "Character" switch dulls the reverb return.

Decrease the input resistors, 470K is absurd.

A Fender 2-jack needs new jacks to get the switching.

0.02u after V1a is NBD.

The 100K-0.02uFd dropper to V1a may be an issue. Gain *rises* below 80Hz. I would use a substantial cap here.

Likewise at V2 Note 7. They must have been trying to use-up odd value paper caps. At today's prices 5uFd-20uFd would be a better choice.

> NOTE 9  1.1M resistor fitted ...Is this setting the Output impedance?

That just bleeds the DC leakage from the cap. The output impedance is the cathode impedance, a few K, plus the cap impedance. It is plenty low. If the 1Meg is not bleeding DC you get a pop when you plug-in. If so, replace the cap. (Again I bet they were using-up odd values found in the factory.)

Have you RECAPPED the main power supply? If not, why are you re-thinking a plan which probably worked OK in 1956?? The 20-10-20-10uFd setup looks OK to my eye, except it is 60 years old! On today's market, I would get a 10-baggie of 22uFd and use them here, also at Note 4 and Note 7.

I would also assume the 35u cathode caps are dried-out husks, and get a bag of 33uFd. Though actually I think they should all be 5uFd. This is still full-bass in the gain stages, and shaved-bass (130Hz) on the power driver stage.

If you wind up "forced" to do a full rip-up or socket replacement, 6EU7 is same-as 12AX7 but very different pin-out (and no 12V heat option). Today a 12AX7 is a more practical tube.

[whoops]

Hi PRR,
thanks you so much for your replies

It would help to know if the hum and hi-cut are in the direct path, the reverb, or both. There are two stages in common and 2 not.

Hum is present at the output with or without reverb added to the Dry Signal
Lo-Fi Sound (high cut) is present at the output also with or without reverb, but it's more pronounced on the FX. Changing the stock reverb tank improves that dramatically. I'm trying to improve the unit in this regard if possible besides changing the reverb tank.
Also I'm not using it for guitar most of the time, it's for mixing music, so parts that limit the Freq range based on the guitar signal could be made to limit a big higher for my needs.

Be aware that I know how spring reverbs are supposed to sound, when I say this unit sounds Lo-Fi in stock form, is by comparing it to all the other spring reverbs I used and not comparing it to a 480L Lexicon Digital Reverb.

It clearly was "too bright" once. The "Character" switch dulls the reverb return.

Yes, that character switch is a high cut switch.
I don't know if it was too bright or not at some point, maybe they just thought it was a good idea in 56.
My switch was broken so I wired that directly without going through any cap

Decrease the input resistors, 470K is absurd.
A Fender 2-jack needs new jacks to get the switching.

Yes I also thought that.
I will replace the jacks.
Should I do Hi and Lo switch, or just have one input jack with 33K?

0.02u after V1a is NBD.
The 100K-0.02uFd dropper to V1a may be an issue. Gain *rises* below 80Hz. I would use a substantial cap here.
Likewise at V2 Note 7. They must have been trying to use-up odd value paper caps. At today's prices 5uFd-20uFd would be a better choice.

Sorry PRR, what does "NBD" stands for? I just got "New Bass Day" on the web.

I also thought some of those caps were paper caps,but it seems they are Mylar caps. Most of the caps used are Ceramic caps.
Theres 2x 1uf, 1x 0.05uf and 1x 0.1 Mylar "Standard" branded caps.
PSU caps + 3x 35uf/50V were electrolytic

you can check the mylar "standard" caps here:

http://el34world.com/Forum/index.php?topic=21305.msg225912#msg225912


> NOTE 9  1.1M resistor fitted ...Is this setting the Output impedance?

That just bleeds the DC leakage from the cap. The output impedance is the cathode impedance, a few K, plus the cap impedance. It is plenty low. If the 1Meg is not bleeding DC you get a pop when you plug-in. If so, replace the cap. (Again I bet they were using-up odd values found in the factory.)

Have you RECAPPED the main power supply? If not, why are you re-thinking a plan which probably worked OK in 1956?? The 20-10-20-10uFd setup looks OK to my eye, except it is 60 years old! On today's market, I would get a 10-baggie of 22uFd and use them here, also at Note 4 and Note 7.

Yes, all Electrolytic Caps were replaced by me when I got the unit. So all the caps in PSU and 3x 35uf 50V were replaced before.

So you would use 22-22-22-22 ?
And then 22u also instead of the 0.02u and 0.1u in Note 4 and Note 7?

I would also assume the 35u cathode caps are dried-out husks, and get a bag of 33uFd. Though actually I think they should all be 5uFd. This is still full-bass in the gain stages, and shaved-bass (130Hz) on the power driver stage.

I actually replaced those 35u/50V for 47u/50v it was the value I had around.
Do you think I should decrease them for 5uf?

If you wind up "forced" to do a full rip-up or socket replacement, 6EU7 is same-as 12AX7 but very different pin-out (and no 12V heat option). Today a 12AX7 is a more practical tube.

Thank you PRR, I have some spare 6EU7 tubes, but thats great to know as there are much more options for 12AX7 nowadays.

I forgot to tell in the first post, that when I got the unit all the tubes were replaced, and the old ones were tested and kept as spares.
All the old tubes tested fine in Hickok tube tester.

[whoops]

NOTE 7 and NOTE 8

I was checking other cathode Follower output designs and all of them seem to use a 1M resistor between signal output and ground, so it seems that value should be better than the 100K drawn in the schematic.

For the Coupling cap, either 0.02u or 0.05u seem to be a low value compared to other circuits where values between 1uf and 5uf are used.

[PRR]

Measure the frequency response and output overload level. "Lo fi" is not much help.

The direct path should probably be <50 to >15KHz with clipping above several V in and out.

The reverb path can't readily be measured because the spring is wonky.

[whoops]

Measure the frequency response and output overload level. "Lo fi" is not much help.

The direct path should probably be <50 to >15KHz with clipping above several V in and out.

The reverb path can't readily be measured because the spring is wonky.

Hi PRR, the unit is dismantled at the moment so I can't measure freq response, also I don't know how to do that. I actually would like to learn in the near future.

I would like to improve what can be improved. Some things are obvious for someone with your knowledge, like the exaggerated grid stopper resistor values that you pointed out.

The Notes points cover most of what I need help into, I will not go much deeper than that myself. (exception will be if someone has suggestions of things that I missed)

Thank You

[2deaf]

The data sheet for section 1 of the 6DR7 shows a grid to plate capacitance of 4.5pf.  As an example, let's say we got a gain of 40 for this section.  That would give you a total input capacitance of 187pf.  A 470K resistor in front of that will give you a cutoff frequency of 1812Hz.  I would call that Lo-Fi.  A 68K resistor in front of 187pf gives a cutoff frequency of 12.5KHz, which I still wouldn't be happy with.  33K gives a cutoff of 25.8KHz, which is good enough for me.

The 470K resistor in front of section 2 is NBD (no big deal) because the gain is extremely small so the Miller Effect is small.  It might have a cutoff frequency of maybe 10KHz which is actually higher than what you would want in order to cut the highs of a reverb driver.  They are getting most of the high rolloff by using a small resistor (1.5K) in series with the tank as opposed to a resistor that is several times larger than the tank impedance.   

[whoops]

The data sheet for section 1 of the 6DR7 shows a grid to plate capacitance of 4.5pf.  As an example, let's say we got a gain of 40 for this section.  That would give you a total input capacitance of 187pf.  A 470K resistor in front of that will give you a cutoff frequency of 1812Hz.  I would call that Lo-Fi.  A 68K resistor in front of 187pf gives a cutoff frequency of 12.5KHz, which I still wouldn't be happy with.  33K gives a cutoff of 25.8KHz, which is good enough for me.

Thank you, that makes totally sense and actually might be the reason for reduced treble of the unit.
I will just use one input jack, with 33K. should I use 1M to ground or 2.2M?

The 470K resistor in front of section 2 is NBD (no big deal) because the gain is extremely small so the Miller Effect is small.  It might have a cutoff frequency of maybe 10KHz which is actually higher than what you would want in order to cut the highs of a reverb driver.  They are getting most of the high rolloff by using a small resistor (1.5K) in series with the tank as opposed to a resistor that is several times larger than the tank impedance.

Might be nice to try a Pot instead of the 1.5K resistor to tweak the high cut of the reverb driver?
Once satisfied I could just measure the pot and install a resistor of the same value.
This made me remember that I should get or build a resistor decade box

Thanks 2deaf, any other suggestions are welcome

[2deaf]

The stock reverb tank is a small cardbox tank, I tried to replace it for an OC electronics tank I had around and the sound improved dramatically, more High end and reverb effect.

That looks like a metal tank suspended in a cardboard box to me.  Every time I have seen the letter "C" on an old tank, it has been the equivalent to a modern type F tank (4F, 8F, or 9F).  The existing circuit will work very well with a type F tank.

If the output coil has an inductance of 358mH, a .02uf capacitor would have a resonant frequency with it of about 1.9KHz.  The output of the coil will increase with frequency below 1.9KHz with a sharp spike at 1.9KHz.  As the frequency increases beyond 1.9KHz, the output plummets dramatically.  This would probably give a shrill character to the reverb.

A 1.0uf capacitor has a resonant frequency with a 358mH coil of around 266Hz.  It has nowhere near the spike that higher resonant frequencies have, but it still dramatically decreases the output at frequencies above the resonant frequency.  This should give a very dull character to the reverb.     

[2deaf]

should I use 1M to ground or 2.2M?

I personally would use a 1M.

Might be nice to try a Pot instead of the 1.5K resistor to tweak the high cut of the reverb driver?

That 1.5K resistor also sets the low frequency roll-off along with the 1.0uf coupling capacitor.

[2deaf]

I was checking other cathode Follower output designs and all of them seem to use a 1M resistor between signal output and ground, so it seems that value should be better than the 100K drawn in the schematic.

For the Coupling cap, either 0.02u or 0.05u seem to be a low value compared to other circuits where values between 1uf and 5uf are used.

Those coupling capacitors are fine for a 1M load, but if you plug into something with a 100K input impedance, you will lose some lows.  Those higher values (1uf, 5uf) are used to provide a low impedance path for 50/60Hz hum that patch cords might pick up.  I recently found out that a 1M output resistor allows a high DC voltage to linger on the output while the large coupling capacitors charge during initial power up.

I also think that you should take the output from the cathode itself instead of the junction of the 2.2K and 100K resistors.  This will give you the lowest output impedance and will help with high frequency loss when using long patch cords.

[whoops]

That looks like a metal tank suspended in a cardboard box to me.  Every time I have seen the letter "C" on an old tank, it has been the equivalent to a modern type F tank (4F, 8F, or 9F).  The existing circuit will work very well with a type F tank.

Yes, you're totally right. It's a Gibbs metal tank suspended inside a cardboard box (I will rectify my previous post).




I measured the DC resistance of the Gibbs original tank:

Input Grounded - DC 251ohms, so it should be aprox 2.5K impedance
Output Grounded - DC 257 ohms, so it should be aprox 2.5K impedance

I don't have an inductance meter, but will try to get one.

The OC Electronics reverb tank that I tried in the unit is:

Input isolated - DC 1.7ohms - aprox 10ohms impedance
Output Grounded - DC 204ohms - aprox 2K impedance

Well even with the input ground isolated and much lower input impedance the OC tank works very well on the unit, the low impedance input of this tank might be the reason why the reverb has much more treble with this tank installed instead of the dull sound I get with the Gibbs unit.
Actually with the OC it doesn't sound too bright to me, it sounds normal, with the Gibbs it sounds Dull.

[2deaf]

I measured the DC resistance of the Gibbs original tank:
Input Grounded - DC 251ohms, so it should be aprox 2.5Kohm impedance

The DC resistance is handy to get you in the right ballpark, but you cannot use it directly to determine the inductance.  I am reasonably certain that a 4F tank has the same coil as an 8F tank and I know for sure that they have the same DC resistance, yet an 8F has a significantly higher impedance than a 4F.  There are other factors that affect the inductance such as the cross section area and length of the flux path.

The published impedance for a 4F tank is 1475 ohms.  The next closest tank is a 4E at 58 ohms.  Your ballpark is obviously a type F.

I don't have an inductance meter, but will try to get one.

I have an inductance meter, but it malfunctions with inductances that high.  I used a resonance method to determine the inductance of type F tanks and they measure out to within 10% of the published value. 

You don't really need to know the actual inductance of the Gibbs because your only choice for modern tanks in that ballpark are type F tanks.   

[whoops]

Those coupling capacitors are fine for a 1M load, but if you plug into something with a 100K input impedance, you will lose some lows.  Those higher values (1uf, 5uf) are used to provide a low impedance path for 50/60Hz hum that patch cords might pick up.  I recently found out that a 1M output resistor allows a high DC voltage to linger on the output while the large coupling capacitors charge during initial power up.

So I can increase that value just to be safe like 1uf or 2.2uf.

I also think that you should take the output from the cathode itself instead of the junction of the 2.2K and 100K resistors.  This will give you the lowest output impedance and will help with high frequency loss when using long patch cords.

I could do that, but I will not use long chords with this unit. When I use it in the Studio it's a really small Jack Link Cable into an Hi-Z input, this could be a DI, a Mic Pre, or a balancing device.
If I use it with Guitar into an amplifier the output cable will not be longer than 1.5meter (5 foot).

I was reading before on cathode follower section of Valve Wizard website, they recommend taking the output from the junction of the 2 resistors instead of straight from the cathod, these are the reasons given:

"('Out2'). I prefer the latter because this allows the bias resistor to do double-duty by isolating the valve from load capacitance somewhat, which tends to improve stability. Yes, a cathode follower can oscillate at very high frequencies if you're not careful, particularly with capacitive loads like cables."

http://www.valvewizard.co.uk/accf.html

Thank you so much for your input and explanations 2deaf

[whoops]

The DC resistance is handy to get you in the right ballpark, but you cannot use it directly to determine the inductance.

I used the DC resistance measurements as a way of getting approximate impedance values for both inputs and outputs, following the advise and tables in this page:

https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared

The tables show an approximate DC resistance measurement of the tanks in/out for each impedance


You are totally right that F type modern tanks are the closest to the original tank input impedance, but as I really prefer the sound with the OC electronics tank that has a much lower input impedance it really makes me think that or I shouldn't match the input impedance of the original tank, I should get a tank witch actually lower input impedance. Or that the values of the 1uf + 1.5K resistor need to be tweaked in order to decrease the High-end roll off.

Until now I thought that the original tank was bad quality and low fi, but might just be an impedance issue, and that the OC Elect tank lower input impedance works much better with the 1.5K series resistor.

[2deaf]

So I can increase that value just to be safe like 1uf or 2.2uf.

Short cables don't have a hum problem, so you don't need to go that big.  Your consideration here is the resulting RC high pass filter.  If 100K is the lowest total impedance that the output will see, 0.1uf is sufficient.  Also, a 0.1uf capacitor avoids most of the initial charging problem.

I was reading before on cathode follower section of Valve Wizard website . . .

I'm not going to argue with that guy, so disregard my comments.

[whoops]

I was reading before on cathode follower section of Valve Wizard website . . .

I'm not going to argue with that guy, so disregard my comments.

Sorry, Im just learning.You will not argue because he is knowledgeable or because you might not agree with him?

[whoops]

I've just edited the Notes questions post with the advice given in this thread. I will keep updating it as to keep track of what I'm going to do in the end, and also to inform possible new readers of the present situation.

At the moment there's just 2 points, probably easy, that we didn't discuss it's Note 6 and Note 10.
Could you help me with that so it's out of the way?

NOTE 6
There's another 100K resistor in the circuit parallel to the one drawn in the schematic.
I don't know if it was a factory assembly error, but it might be.
The 2 paralleled 100K resistors form a 50K resistor.
What's your opinion, should I use 100K or 50K here?

NOTE 10
The filtering arrangement was different than the schematic when I got the unit, and I don't remember how it was originally as it was a long time ago. At that time I didn't notice there was no dot between at this connection, so I rewired the circuit like the dot was there.
Is the missing dot in the schematic a mistake like I assumed?

Thank you

[shooter]

#10 should be a dot

#6 Might want to go with 100k and if you don't like it || it, lots easier than If you wanted to series it

[2deaf]

Sorry, Im just learning.You will not argue because he is knowledgeable or because you might not agree with him?

Merlin is like an internationally recognized authority on audio circuits.  If I ever did disagree with him, I would certainly keep it to myself.

[whoops]

Your consideration here is the resulting RC high pass filter.  If 100K is the lowest total impedance that the output will see, 0.1uf is sufficient.  Also, a 0.1uf capacitor avoids most of the initial charging problem.

Thanks, I will keep the original capacitor here then.

[2deaf]

NOTE 6
There's another 100K resistor in the circuit parallel to the one drawn in the schematic.
I don't know if it was a factory assembly error, but it might be.
The 2 paralleled 100K resistors form a 50K resistor.
What's your opinion, should I use 100K or 50K here?

50K will increase the voltage to both 6EU7's thereby increasing the headroom.  Neither one of those stages is even coming close to having a headroom problem.  The reverb recovery stage is dealing with a signal on the magnitude of 10mV.  The cathode stage is dealing with line level signals.  Both stages are drawing very little current so that the difference in the voltage drop from 100K to 50K isn't going to be very big, maybe 50V or less depending on the B+ voltage.  Flip a coin. 

[whoops]

#10 should be a dot

Thank you so much shooter

[PRR]

> take the output from the cathode itself

The difference is extremely small. If you are that close to happy/unhappy, you probably really want a beefier driver.

I don't agree with all Merlin's deep thinking, and am not sure his point makes a big difference either. I'd probably take whichever point made the wiring easier.

In THIS case the output is clearly GUITAR LEVEL. A Volt or less into 10 feet of cable. If you have to go further, that's what DI Boxes were invented for, go out on mic cable as far as you want into a mic input. If you want a Line Level, now we are lusting for some gain, a 10V output, and 10k to 600r loads. This is a couple more tubes or an op-amp.

For now, leave this area more or less as-is. The perceived "lo-fi" and the hum are the key problems.

Frequency Response is a key skill for an audio technician. Used to be the 2nd and 3rd instruments a tech got (after a good DC meter) were a signal generator and a good AC/Audio voltmeter. These are STILL essential tools in this iPad age. While you can do "sound card checks" on PC with an external device in the loopback, that's a limited set of checks. Beautiful graphs, not the info we need.

[whoops]

Both stages are drawing very little current so that the difference in the voltage drop from 100K to 50K isn't going to be very big, maybe 50V or less depending on the B+ voltage.  Flip a coin.

LOLOL
I was going to do that, just flip a coin like you suggested. But meanwhile I found a reverb unit branded "Airline" that seemed to be the same unit as mine but different brand and cosmetics.

I found the schematic and actually the Airline schematic is closer to my circuit than the Gretsch schematic.
This Airline schematic was probably released later than the Gretsch, and there's some nice things there.

In the Airline reverb schematic that resistor is actually 47K, so it makes sense the 50K, probably the value was changed at some later revision and at the factory they found would be more practical to just add two 100K resistors in parallel.
Probably the 100K resistor stock was high.

I will post the schematic here.

[whoops]

Thanks PRR.

I would like to have some Frequency Response pro measuring equipment, but this is an hobbie to me.
I'm a Sound Engineer that got hooked into audio electronics with GroupDIY, I learned a lot with you and everyone at GroupDIY but sometimes it's a steep learning curve as I cannot dedicate all my time to it. But I love this hobbie and love learning .

I already have a nice Fluke Multimeter, hopefully I will be able to fix a nice Oscilloscope that was a gift and be able to learn and use the Scope.
As for Frequency Response I guess for the needs of this thread even some simple tests with sound card and free software would be helpful. I will try to learn and dedicate some time to it and post the results here.
Just can't do it at the moment as the unit is dismantled because I was tracing the circuit against the schematic and lifting resistor legs to measure their value.

As for the perceived "Lo-Fi" and Hum being the key problems, yes you are right, but as this is a restoration and tweaking project and as I'm using the unit mainly for Mixing and not so much for guitar any suggestions you might have that could improve the stock circuit are always welcome.

For the Hum part, I think lowering the value of the grid stopper resistors, increasing the filtering in Note 4 and Note 7, rewiring the heaters with artificial centre tap and choose a better location for the Heater wires will possible improve a lot the hum I was complaining too.
Maybe also replacing some connections with shielding wire.

Let's see how this first phase goes and I will let you know the findings and post the results.

Thanks

[whoops]

here is the Airline GVC-9019A Schematic




It seems an improved version of the Gretsch Schematic:

- DOT on R24/R25 junction
- C2, C11, C12, R21 and R22 have the same values I found in my circuit that were different than the Gretsch schematic
- 6EU7 Pins are corrected

Differences to my circuit:
- C3 - 0.02 on my circuit - 0.05 on both Gretsch and Airline schematics (this is insignificant)
- R18 - 1.1M on my circuit - 6.8M on the Airline
- R13 - non-existent on my circuit or on the Gretsch schematic, this is a 2.2M resistor added on the Airline model

[whoops]

You are totally right that F type modern tanks are the closest to the original tank input impedance, but as I really prefer the sound with the OC electronics tank that has a much lower input impedance it really makes me think that or I shouldn't match the input impedance of the original tank, I should get a tank witch actually lower input impedance. Or that the values of the 1uf + 1.5K resistor need to be tweaked in order to decrease the High-end roll off.

Until now I thought that the original tank was bad quality and low fi, but might just be an impedance issue, and that the OC Elect tank lower input impedance works much better with the 1.5K series resistor.

Could R13, the 2.2M resistor added probably later in the Airline model be related to why I found a lower input impedance (10r) reverb tank sounded better in the circuit than the stock higher input impedance (aprox. 2,5k) tank?
Or that resistor is there just to preventing a click/pop sound when using the footswitch to switch the FX in and out?

[whoops]

Hi, I will perform the advised mods this weekend. Thank you all for the support.


Could someone please help me in my last post?
Is it possible that actually a lower input impedance tank will work better with this circuit thant the higher impedance stock tank?


Maybe the manufacturer just used at the time the tanks that they had already available at the warehouse without respecting the input impedance the circuit would be more eficient in driving.


Thank you

[terminalgs]

Until now I thought that the original tank was bad quality and low fi, but might just be an impedance issue, and that the OC Elect tank lower input impedance works much better with the 1.5K series resistor.

The tank's impedance rises in a linear fashion as the frequency rises. As the frequency of your input signal goes up, the Zin of the tank also goes up.  So your OC tank's 10ohm Z is 10ohm @ some frequency, but at higher frequencies it's higher.  If it sounds good, great!

Could R13, the 2.2M resistor added probably later in the Airline model be related to why I found a lower input impedance (10r) reverb tank sounded better in the circuit than the stock higher input impedance (aprox. 2,5k) tank?
Or that resistor is there just to preventing a click/pop sound when using the footswitch to switch the FX in and out?

I think that's to prevent a click,  2.2M too big to effect impedance.

[whoops]

The tank's impedance rises in a linear fashion as the frequency rises. As the frequency of your input signal goes up, the Zin of the tank also goes up.  So your OC tank's 10ohm Z is 10ohm @ some frequency, but at higher frequencies it's higher.  If it sounds good, great!

Definitely, but I'm just trying to understand if the reason why people don't like the stock tank might be not related to the quality of the tank like other people supposed but to impedance issues with the circuit and the tank's high input impedance.

As I'm thinking in getting a new tank for the unit, that's really relevant to know since if I buy a type F modern reverb to match the stock tank high input impedance I might still have the same problem that I was trying to solve

[whoops]

 I was able to measure both tank's for input and output impedance:

Gibbs original tank:

Input Grounded - DC 251ohms, so it should be aprox 2.5K impedance, 287mH inductance
Output Grounded - DC 257 ohms, so it should be aprox 2.5K impedance, 352mH inductance



The OC Electronics reverb tank that I tried in the unit is:

Input isolated - DC 1.7ohms - aprox 10ohms impedance, 1.12mH inductance
Output Grounded - DC 204ohms - aprox 2K impedance,  302mH inductance

[sluckey]

I would use a hi Z input tank in that cap driven circuit. It may be that old tank is just tired and needs to be put to rest. If that's the case, anything you put in it will probably sound better. A new hi Z tank will probably sound even better than your low Z tank. I would expect it should give you more boing.

I doubt that unit ever sounded like a Fender unit. Valco probably built the Gretsch unit. They had a rep for building 'affordable' gear, even if it had the Gretsch name on it.

[whoops]

I would use a hi Z input tank in that cap driven circuit. It may be that old tank is just tired and needs to be put to rest. If that's the case, anything you put in it will probably sound better. A new hi Z tank will probably sound even better than your low Z tank. I would expect it should give you more boing.

I doubt that unit ever sounded like a Fender unit. Valco probably built the Gretsch unit. They had a rep for building 'affordable' gear, even if it had the Gretsch name on it.

Hello Sluckey, thank you so much for your reply.
I suspected the input impedance because the old tank actually looks in perfect shape and once you take it out of the cardboard box you can see that it seems to actually be better built than Accutronics tanks.
I will do as you and 2deaf suggested and try a Hi Z input tank in this circuit and report the results here, maybe even doing some recordings of the sound with different tanks.
At the moment the higher impedance tank I have available for the tests is a D type accutronics, 310ohm input impedance.
I will get an F type tank (1925ohm in impedance) before I do the tests.
I just wouldn't like to commit to buy one before I had your assurance that this circuit should work well with an HiZ tank and that it was not an original production mistake.

Yes this unit was built by Valco so it was on the affordable side of the market, it will never sound as good as a Fender unit for sure. But doing the small mods and improvements as suggested in this thread, like lowering the grid stopper resistors from 470K to 33K will probably be a nice improvement. Just changing the tank from the stock HiZ to the OC lowZ tank was already a big improvement and make the unit go from unusable to a nice spring reverb, that made be believe that with some tweaks this unit could actually be quite useful for my needs.

Sluckey if see something else that you think could be improved or tweaked for better performance, please let me know.

Thank you so much

[2deaf]

Gibbs original tank:

Input Grounded - DC 251ohms, so it should be aprox 2.5K impedance, 287mH inductance
Output Grounded - DC 257 ohms, so it should be aprox 2.5K impedance, 352mH inductance

The OC Electronics reverb tank that I tried in the unit is:

Input isolated - DC 1.7ohms - aprox 10ohms impedance, 1.12mH inductance
Output Grounded - DC 204ohms - aprox 2K impedance,  302mH inductance

Not that it matters, but I am curious as to how you arrived at the impedance values.  287mH calculates to an impedance of 1803 ohms, 352mH is 2212 ohms, 1.12mH is 7 ohms, and 302mH is 1897 ohms.

[whoops]

Gibbs original tank:

Input Grounded - DC 251ohms, so it should be aprox 2.5K impedance, 287mH inductance
Output Grounded - DC 257 ohms, so it should be aprox 2.5K impedance, 352mH inductance

The OC Electronics reverb tank that I tried in the unit is:

Input isolated - DC 1.7ohms - aprox 10ohms impedance, 1.12mH inductance
Output Grounded - DC 204ohms - aprox 2K impedance,  302mH inductance

Not that it matters, but I am curious as to how you arrived at the impedance values.  287mH calculates to an impedance of 1803 ohms, 352mH is 2212 ohms, 1.12mH is 7 ohms, and 302mH is 1897 ohms.

Hi 2Deaf, I explained that before already.

But it here goes again.

Thank you

The DC resistance is handy to get you in the right ballpark, but you cannot use it directly to determine the inductance.

I used the DC resistance measurements as a way of getting approximate impedance values for both inputs and outputs, following the advise and tables in this page:

https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared

The tables show an approximate DC resistance measurement of the tanks in/out for each impedance

[2deaf]

I just wouldn't like to commit to buy one before I had your assurance that this circuit should work well with an HiZ tank and that it was not an original production mistake.

Healthy skepticism is a good thing.

If you draw the driver out using 300V from the power supply, it looks like it is idling at 32mA.  The RCL load at the AC frequencies that we use is approximately 1.5K with a type "A" tank for L and 1uF for C.  The published nominal current for a type "A" tank is 28mA_rms which is approximately 40mA_peak.  With 1.5K in parallel with 5K, it would take 52mA to get 40mA across the 1.5K.  The 6DR7 will swing 52mA in the positive direction, but with an idle of 32mA it obviously will not swing 52mA in the negative direction.  An 8D tank with a 49.3mH input is still too small to to affect the dominance of the 1.5K resistor in the RCL equation, so the load is still approximately 1.5K.

The 6DR7 will drive an 8 ohm tank, but not to its full potential and not with a symmetrical signal.  It will drive an 8F tank well past the published nominal value and with a much more symmetrical signal.




 

[whoops]

I just wouldn't like to commit to buy one before I had your assurance that this circuit should work well with an HiZ tank and that it was not an original production mistake.

Healthy skepticism is a good thing.

If you draw the driver out using 300V from the power supply, it looks like it is idling at 32mA.  The RCL load at the AC frequencies that we use is approximately 1.5K with a type "A" tank for L and 1uF for C.  The published nominal current for a type "A" tank is 28mA_rms which is approximately 40mA_peak.  With 1.5K in parallel with 5K, it would take 52mA to get 40mA across the 1.5K.  The 6DR7 will swing 52mA in the positive direction, but with an idle of 32mA it obviously will not swing 52mA in the negative direction.  An 8D tank with a 49.3mH input is still too small to to affect the dominance of the 1.5K resistor in the RCL equation, so the load is still approximately 1.5K.

The 6DR7 will drive an 8 ohm tank, but not to its full potential and not with a symmetrical signal.  It will drive an 8F tank well past the published nominal value and with a much more symmetrical signal.

Thank you so much 2deaf, once again.
It's much more clear for me now.

I will do the mods we discussed before and get an 8F type tank. Will do some tests them and let you know the results.

[whoops]

Here is the new schematic with the Mods that will be performed:

[whoops]

Have one last question before doing the Mods and assembling the unit.
It's about the possibility of using shielded cable at some specific places.
The stock unit only has shielded cable in the places traced with RED in this schematic.
I was thinking of adding shielded cable from the input jack to the first tube. And also from the circuit to the input of the reverb tank, and out of the reverb tank until the 6EU7 pin 8. (traced in BLUE in the schematic).
What do you think?
Any other place you think might be beneficial to use shielded cable in this circuit?
Maybe the connection to the Intensity Pot and from the Pot to the C9 as this is the Dry audio signal?
Thanks