Silvertone 1474........

[Jack_Hester]

Well, the filaments should last for quite a while, though I didn't measure this voltage when power is first applied. 

Now I'm back to thinking modifying the driver circuit around the tube, rather than the source follower.

I was looking at the Ampeg R-12-R, and it's use of a 6SN7 triode as the driver.  It has a plate resistance like my 6CG7.  The plate resistor and the cathode resistor and cap are easy to get to.  Using the resistance values (plate resistor, plate resistance, cathode resistor) from that driver, and calculating the impedance presented to my tank (tank being 1475), that gets me awful close to what I have. 

Does this appear to be a likely solution?  Comments please.  Time to call it quits for the day.  Have a good one.

Jack

[sluckey]

Well, the filaments should last for quite a while, though I didn't measure this voltage when power is first applied.

Voltage will be zero until the output tubes warm up and start conducting.

[Jack_Hester]

Should make for a good soft start.

Jack

[Jack_Hester]

Attached is the Reverb circuit, in it's stock configuration.  But, with Driver cathode capacitor removed, and the plate resistor and cathode resistor values changed to match that of the Ampeg R-12-R.  I'm thinking that I may need to move that B+ source to B+3.


Comments, please.  Thanks. 

Jack

Note: See Reply #69 for the Reverb circuit, as stated above.  But, with proposed additional Driver tube stage.

[jazbo8]

Attached is the Reverb circuit, in it's stock configuration.  But, with Driver cathode capacitor removed, and the plate resistor and cathode resistor values changed to match that of the Ampeg R-12-R.
Jack

I am pretty sure that the Ampeg has a standard spring reverb tank, and not a piezo element, so the circuit is likely to be ill-suited for the 1474. The difference in the drive requirements can be found here (and read the comments on the piezo tank while you are at it).

[Jack_Hester]

I am pretty sure that the Ampeg has a standard spring reverb tank, and not a piezo element, so the circuit is likely to be ill-suited for the 1474. The difference in the drive requirements can be found here (and read the comments on the piezo tank while you are at it).

You're right about the spring tank in the Ampeg being standard.  I can't see any specs on mine (Reverberocket), as I have not removed it for a closer look. 

Can't see how the 1474 would be ill suited.  I'm satisfied that I would have to change the plate and cathode resistors, and move the B+ upstream, closer to the source (B+3), to handle the current. 

I read the thread from your link.  Unless I'm not seeing something in the comparison between my tube (6CG7) and the 6SN7 that the R-12-R is using, it appears to be able to provide the current needed, by the specs. 

Look at another Reverberocket schematic that I've attached.  It uses a 6U10 (section 1) with the same specs, as well.  Appears to be the same as the 6CG7 and the 6SN7.  Same plate and cathode resistor used. 

The Gemini I (G-12) uses a 6CG7, but differences are a 10K plate resistor, and 330R cathode resistor bypassed by a 25uf-25v cap.  Tells me that there is a different tank from the Reverberocket. 

Anyway, a couple resistor changes makes my 1474 look workable with the right tank.  At this point, a 2.2K-5W plate resistor, and a 330R-2W cathode resistor (unbypassed) puts me closer to the 1475 ohm impedance that I'm shooting for.  The tank that I'm attempting to use is a 4FB3D1B 

I haven't given up on the solid state route, yet.  Just back to exploring a tube-only solution, without adding another tube. 

Jack

[jazbo8]

I must have missed it along the line, I thought you were still using the piezo element... the comment about the suitability of the Ampeg circuit was with regard to the voltage requirement as mentioned in the link I posted earlier... sorry for any confusion it may have caused. So the Ampeg circuit should be a good mate for the new tank that you plan to install.

[Jack_Hester]

That's ok.  Little by little, I'll add detail as to what I'm after, as the questions arise.  Keep looking.  You may see something better to try. 

I don't have the components needed (resistors), as shown in the Reverberocket drawing.  But, I can round them up in short order.  I have to run over to Durham (NC) in the morning.  I can check with Bull City Sound, to see if they might have them on hand. 

Jack

[Jack_Hester]

So, I've been a bit impatient.  I looked at my components, and ran some numbers on what I had on hand. 

This is what I came up with: 

470 ll 1K = 319.7 ohms (these are 1W resistors) - inserted this combination in place of the cathode resistor and bypass cap. 

4.7K ll 4.7K ll 47K = 2238.1 ohms (these are 3W resistors) - inserted this combination in place of the plate resistor. 

Also, I put a 1R-1W resistor in series with the cathode combination. 

Note: All resistors mounted to RS breadboard, and jumpered to the amp. 

Warmed up the amp and took some readings:

B+3       = 187vDC
V2-pin6 = 160vDC
V2-pin8 = 4vDC
Voltage across the 1 ohm resistor = 13.8mV (tube spec sheet says I'm good for 22mA cathode current).

Tried it out with the guitar.  Guitar volume is normal, but I just can hear a very, very subtle echo.  First time that this has happened.  Ear at the speaker to hear it, kind of weak.  Tapped the tank, and it is plenty loud.  Recovery working good. 

Suggestions please, as to value changes, or if I'm headed in the right direction.  Feeling a bit better about this mod. 

Jack

[jazbo8]

Try removing R29 (68k) and replacing it with a capacitor (say 0.1-0.5uF), the reverb tank is not getting enough drive with the resistor there, if the reverb level is still too low, try putting back the cathode bypass capacitor (from pin 8 to ground).

[Jack_Hester]

Try removing R29 (68k) and replacing it with a capacitor (say 0.1-0.5uF), the reverb tank is not getting enough drive with the resistor there, if the reverb level is still too low, try putting back the cathode bypass capacitor (from pin 8 to ground).

I put an alligator jumper across R29, and did not replace it with a cap.  That didn't work, for sure.  All the Bass disappeared and the sound was very thin.  The 10uf-150v bypass that I removed would not work on the breadboard.  Leads too short.  I put a 25uf-25v as a cathode bypass.  No change that I can hear.  I'll see if I can dig out a cap to replace R29. 

Just out of curiosity, I measured the DC resistance of the tank input.  219 ohms.  Good, according to the spec sheet for this tank.  I'll be back.

Jack

[Jack_Hester]

Replaced R29 with a .1uf-400v cap.  Warmed it up and gave it a try.  Maybe a small increase, but I don't hear well enough to say for sure.  Got the ringing thing going on in my ears.  Sounds as good as it did with the 68K in place. 

Recovery still in good, loud working order. 

Just to see how much difference the bypass cap made, I pulled it from the breadboard.  Overall volume dropped a little.  I put it back.  Still running just shy of 14mA at the cathode. 

Good tests.  Just got to scratch my head a bit more.  Thanks for suggesting.  Sooner or later, the solution will show up. 

Jack

[Jack_Hester]

While I'm thinking, I decided to swap out V2 with a 6N2P, which has a mu of 100.  Sound became thinner on that channel.  Put the 6N5P back in, and swapped V4 to a 6N2P.  That sounded a little better.  Amp a little louder, and I can hear the echo only a little better.  Not much.  I'll swap V4 back to a 6N5P, and leave it that way until I have a stronger Reverb.  Then, I can experiment then. 

Jack

[PRR]

> a very slow warmup time

11 seconds for the big tubes to warm-up.

Then *another* 11 seconds for big-tube current to heat the little tubes.

20+ seconds to first sound.
_________________________________________

> PT's might not have have enough filament amperage

Doubt it. It's a slick-trick for *clean* DC power to a few key heaters.

[jazbo8]

Replaced R29 with a .1uf-400v cap.  Warmed it up and gave it a try.  Maybe a small increase, but I don't hear well enough to say for sure.  Got the ringing thing going on in my ears.  Sounds as good as it did with the 68K in place. 
Jack

Had a closer look at the two schematics, and I think the problem lies in the different gain of the 1474 vs. the Ampeg. The Ampeg has two gain stages before the 6CG7 reverb driver, while the 1474 only has one, so the drive signal is too low for the reverb tank. One obvious mod is to jumper the two channels together, i.e., take the output from Channel One and run it into Channel Two (may be via a switch).

[Jack_Hester]

> a very slow warmup time

11 seconds for the big tubes to warm-up.

Then *another* 11 seconds for big-tube current to heat the little tubes.

20+ seconds to first sound.
_________________________________________

Pretty close to actual.  I timed it right at 30 seconds.

Jack

[Jack_Hester]

I noticed a discrepancy in my drawing.  I'm sure there may be more.  The one that stands out is that my drawing shows Channel 2 pre-amp connecting to the junction of C8/R16, by means of C7.  It does not connect there.  C7 connects directly to V2-pin7 (Reverb Driver).  I can't see it clearly, the way the chassis is laying, but the output of the Reverb pot connects to the junction of C8/R16, on the Tone Stack.  I'll fix the drawing later, and post the correction.

Had a closer look at the two schematics, and I think the problem lies in the different gain of the 1474 vs. the Ampeg. The Ampeg has two gain stages before the 6CG7 reverb driver, while the 1474 only has one, so the drive signal is too low for the reverb tank. One obvious mod is to jumper the two channels together, i.e., take the output from Channel One and run it into Channel Two (may be via a switch).

I believe that you've figured it out.  My Oliver G-300 has two pre-amp stages and another stage within the 6DR7 (total of 3), before the Driver stage.  My Ampeg R-12-R has two before the Driver Stage.  This one has only the one pre-amp. 

I think that I'll leave Channel 1 as-is, and now try to figure the easiest way to bump the gain up on Channel 2, without going the transformer route on the tank.  And, keeping it all on Ch2.  I'll button up Ch2 with the correct components (2.2K on plate, 330R on cathode), and proceed from there.  More breadboarding.

Jack

[terminalgs]

If gain is an issue, why not get the signal from V1's plate?





Add C2,   use R1 & R2 to attenuate the signal to your liking.  Make the sum of R1+R2 be high, like 470K-1M so C2 doesn't change the tone of the signal headed to the tone stack.


If you still don't have enough gain to suit you,  of your three mix resistors (make all 330K), lower the R5 mix resistor to up it's level in the mix.


Don't worry about the tone not getting shaped before hand, reverb is too murky anyhow. if you want,  I'd add a cap to ground  after the recovery or driver (like the ampeg) to roll off the highs and be done with it...

[Jack_Hester]

The more I've been thinking about this, the more complicated it seems to get, trying to raise the gain enough to drive the tank.  Originally, I wanted to mod, without doing much.  That ain't gonna happen. 

I didn't read your post before coming to a decision.  I really want to keep the Reverb in Channel 2, and let Channel 1 be clean.  I suppose I should have said that early on.  Anyway, I didn't want to add another tube, so I explored the MOSFET route.  From what I'm reading, I'm not going to get an IRF820 to work as a gain stage, and I don't have any other MOSFET's/JFET's. 

So, I resigned myself to the fact that another tube was the best way to go, and started looking at them again.  I realized after looking the schematic over again, I stopped at the Tremolo circuit.  I have an abundance of 6AU6's.  I went back and looked at the chassis around V2, and decided that I could squeeze in a small 7-pin socket near V1 and V2, without crowding anything more than it is. 

I went through some of my smaller project schematics and came up with a possible extra stage for the driver, implementing a little 6AU6. 

Jack

[Jack_Hester]

Well, this has be an enjoyable experience in gain.  I added the little 6AU6 (V11) to the Reverb Driver circuit.  If it has an affect on driving the spring tank, I can't hear it.  I have a most wonderful overdrive circuit, and I can't tell if there's echo.  I actually enjoyed playing with it so much that I had to make myself remember that I was trying to get some Reverb out of this amp. 

So, I'll study it some more, but I'm thinking that I will keep this mod.  I can turn the guitar volume way down and the whole channel is clean.  I may have to add a 1M-A gain pot in front of V11 for this purpose.  It was a very interesting turn of events.  Channel 1 remains a downright respectable clean channel. 

I used a shielded socket base for V11, but I need see if I have a shield for it.  I believe that it needs one.  More to come. 

Jack

[Jack_Hester]

Added the 1M-A gain pot and that was just what it needed to tame it down.  Gives me clean to overdrive, and lots in between.  Found a suitable spot beside the footswitch in the back, and mounted it.  Shielded wire around to the connection points.  Updated the drawing in the previous post, to reflect the addition. 

So now, back to pondering the spring tank and what else I may do to wake it up more.  I will say that by adjusting this new gain pot to the point where it just starts to break a bit, I can hear echo in the speakers without putting my ear next to them.  Still a ways to go, but progress and fun all at the same time. 

Jack

[Jack_Hester]

Took a complete set of voltage readings this morning.  They are attached to the drawing.  Moved the modified Reverb circuit up to the main drawing, and the stock configuration below it.  You'll have to zoom in to read them, as I made them the same resolution as the drawing. 

I'm wondering about the voltage on V3-pin7.  Should I expect to see 8.5vDC there?

Jack

[Jack_Hester]

I should also note that C15 (.01uf) that connects Pin1 to Pin7 is new.

Jack

[Platefire]

Jack

  I been keeping an eye on this post. Years ago I use to belong to a Silvertone Forum that's now defunct but correcting that spacey ST reverb was often a subject on that forum. So I'm watching with interest hoping you get to the bottom of it and get a great sounding reverb out of it. I'm sure that is a recipe that a lot of folk would be interested to know. Platefire

[Jack_Hester]

Thanks, buddy.  I've cleaned up the cabinet, and mounted the chassis with new screws.  I don't think this amp has ever had the attention that I've given it, in the past week or so.  It looks really good.  I've put it aside, until the new 'C' tank gets here from Fliptops.  I had a few Ampeg parts to order, so I decided that a new tank dedicated to this project was in order.  The tanks that I've tried (only 2 of them) have been laying around for some time.  They are both supposed to be 1475 ohm (in) and 2250 ohm (out), but until now, never tested.  Once I resolve this issue, I'm going to get a woodworking buddy to make a small enclosure to match the cab, put feet on it, and either sit it on top or underneath the main cabinet. 

Anyway, the rest of the amp is in downright decent working condition.  But, the chassis will go back on the bench, when I have some cables made up and the new tank in hand. 

I sure would have liked to have spent time in a Silvertone forum.  Thanks for watching.  Have a good one.

Jack