Fender Dual Showman Tremolo clicking and speed issues

[Mr Johnny Birchwood]

Hi guys, I've got a '74 Dual Showman Reverb on the bench right now. Came in to get new power tubes and re-capped filter and coupling caps. Owner wanted the amp blackfaced so that's what I did. Adjustable bias instead of balance. Also added NFB disconnect using the push-pull master volume switch(that sucked) and using the master volume pot as a reverb dwell pot. The issue I'm having is with the Neon tremolo. The schematic from fender lists a 12AX7 in V5 but the factory tube basing diagram inside the amp lists a 12AY7 in V5. The circuit is wired identical to the 12AX7 version of the circuit, and according to Fender they never made a DSR with 12AY7 in V5. The tremolo has a noticeable ticking, clicking sound when turning on and off. Is there circuit mod or even a replacement for the neon bulb that will yield a softer/more gradual on/off cycle? Owner wants me to slow down the tremolo(which I'm pretty confident I can do) and wants to get rid of the tick and soften the effect. My thought was to maybe even get rid of the neon bulb altogether and go to a power-tube bias type LFO. Any thoughts, experience, or guidance would be much appreciated.

Converting to the Twin Reverb AB763 ckt btw..

[g1]

Never seen or heard of a Fender using a 12AY7 for the tremolo. There were plenty of amps with the wrong sticker in them, they sometimes used something "close" if they didn't have the exact one. However I'm not aware of any stickers that showed a 12AY7 for V5 unless there was a typo that I'm not aware of. Any chance there is a wrinkle in the sticker or poor printing?
Here is the standard fix for trem. ticking:
Fender Tech Notes - Vibrato Ticking Fix.pdf

[Mr Johnny Birchwood]

No, it is clearly printed as 12AY7 in V5. I can take a picture of it later and post it if you'd like. Thanx for the link, I'm gonna get back to the DSR later today. Needless to say, the 12AY7 sticker had myself and Fender Support pretty confused there for a bit. I guess it's just another CBS-era anomaly...

[Enzo]

Forget the stickers, stick with the schematics.

[Mr Johnny Birchwood]

Tremolo issue was resolved with lead dress. Slowed it way down with bigger caps, .047, .02, .02. The current issue is the reverb. Despite some circuit mods to increase drive, reverb is still very weak. I took the pan out of the bag and I'm not sure if it is the correct unit. It is a US made Accutronics tank, 9-inch 3-spring with an 800 ohm input transducer and 2575 ohm output transducer. The transducer mounting plates are bent pretty badly as it appears that somebody attempted a repair on the unit. There is a large glob of soft, clear silicone adhesive on the leads where they connect to the transducer coil. The transformer part number from the silverface schematic is 125A 20A. Anybody know if the 800ohm, 9-inch, 3-spring tank was stock to the silverface Dual Showman? Customer will want an upgrade so I just want to make sure I match the input impedance correctly before I order the 17-inch, long-decay, 3-spring unit. Customer prefers a VERY wet reverb sound.

thanx

[g1]

All the older Fender's used 4AB3C1B from around '63 till the '90's. Input impedance 8ohm (reads about 1ohm), output impedance 2250 (reads about 215ohm).
So what you have was not stock, no 3 spring (or 9 inch) would be. The input impedance mismatch is probably why it sounds weak.
3 spring equivalent of stock unit would be 9AB3C1B. You can get a very wet sound from the stock unit, many prefer the stock sound over the 3 spring.

[Mr Johnny Birchwood]

Thanx g-one. I had pretty much reached that same conclusion myself, but sometimes you just never know what you are gonna find in a Silverface. The one thing that gave me some doubt is the Silverface Dual Showman schematic from Fender lists the transformer part number as 125A 20A, and the 25k-8ohm replacement part is listed as a replacement for the 125A 20B transformer. It was also vertically mounted in a 17inch bag, which would be preferable to the horizontal mount if I can find a suitable vertical mount replacement.

The reverb transformer primary ohms out at about 1.7k on a DC meter which is what I would expect for the 25k primary. What would the secondary of the 25k-8ohm reverb transformer read on the meter? I assume it would be somewhere close to 1 ohm. The input coil of the tank reads 60ohms DC resistance, so I would assume that the secondary of a 25k-600/800ohm transformer would read about the same 60ohms as the transducer.

As for the 3-spring tank, that's what the customer THINKS he wants. I've had good results with both the 2 and 3 spring tanks, and I usually add a dwell/drive pot(like I did with this DSR) when I use a non-stock tank, so that the player can dial in the effect they want. For more drive I just use a slightly bigger coupling cap, up to 2200pf, in place of the 500pf. How do you deal with a customer that KNOWS what they want, even if what they want isn't really what they want?

[Tom Phillips]

... How do you deal with a customer that KNOWS what they want, even if what they want isn't really what they want?

That's a tricky situation. My approach is to ask them what sound they want. I.e. what sonic effect do they expect from the parts change or mod they have requested. I also ask where they got the idea? Was it past experience, a forum discussion or just plane old internet lore? Based on the answers you then need to be able to give the customer the information they need to make their final decision.It's always important to determine if the change is a band aid compensating for some other problem in the amp or a true "improvement."

Everybody isn't "calibrated" the same. It's best to listen to the customer play the amp and describe what they want. That can eat up A LOT of shop time though. One person's perfect tone could be another person's harsh ice pick sound. Plus you have different personalities to deal with. This list goes on.

HTH,
Tom

[Mr Johnny Birchwood]

I know this guy pretty well, played in a couple bands with him in the past. Bottom line is: Either the 2-spring or 3-spring 17 inch tank will be an upgrade from the non-stock replacement of unknown origin. After some consultation he told me to use my best judgement in ordering the replacement. I decided to go with the 3-spring tank, as the low-end response will be a little more articulated than the 2-spring. With the dwell pot and the bigger coupling cap he will be able to dial in what he wants. I feel that the 3-spring will sound better when played VERY wet, and this guy likes his reverb VERY VERY wet. After some consideration I think that in this case he is right, though not necessarily for the right reasons.

It's a strange phenomena I guess, but if a player BELIEVES that his amp will sound better with a different part, then he will play with more CONFIDENCE, which actually will make his playing(and therefore the amp) sound better. It may not be the part itself, but the psychological effect of having some 'magic' part in the amp may actually produce the desired results in a roundabout audio-psychosomatic sorta way... Who am I to argue?

[Enzo]

2 spring, three spring, give the customer what he wants, either will sound and work fine, so there is no loss.

It is more useful to use the part number rather than impedances 9AB2C1B tells me everything I need to know, and it sounds to me like you are trying to use a 4EB2C1B where a 4AB2C1B belongs.


If your Fender reverb drive transformer is not open or shorted, it is OK, the impedance won;t be wrong. All transformer driven reverbs in fenders take an AB pan. 4AB.... or 9AB.... take your pick. Just not 4EB.... or 9EB....


Transformers and transducers all run on turns rations, not resistance. The resistance of a winding depends a LOT on the size of its wire. I do not expect both tansducer and transformer to have the same DC resistance.

[Mr Johnny Birchwood]

The 4EB... was in the amp when it came in. Some hack-job previous 'repair.' I ordered the 9AB after double checking the transformer winding. DC resistance of a transformer or transducer winding is not the best way to determine the spec, but an 800ohm winding will read different than an 8 ohm winding. Transformer is stock unit and it's good, so yeah the 9AB will work. Previous hack-job 'repairs' make it necessary to inspect everything, even if it seems obvious. The 800 ohm transducer(nominal 1khz impedance) reads 60ohms DCR on a DMM, which is how I figured out what it was.

[Enzo]

The Accutronics web site has reverb specs, and they are also other places. The specs include resistance, but the letter code chart is there too, so if it says AB or EB or whatever, you can skip the readinjgs. I always use my ohm meter, but all i am doing is checking for continuity or open.

[g1]

Here's some info covering the codes, impedances and DC resistance readings, as well as some common circuits and a subjective comparison of different brands of tanks:
Spring Reverb Tanks Explained and Compared | Amplified Parts

[Mr Johnny Birchwood]

If there is no code on the unit in question, then you have to use other methods to determine what it is. Impedance and especially inductance are very tricky phenomena to understand and to quantify without special tools and knowledge(that I don't have). Impedance ratings are of course a nominal impedance, usually a 1khz nominal rating, and there is much more that goes into the construction or selection of a transformer or transducer than just the nominal impedance. That's the tricky part of reverb drive circuitry, the impedance and therefore input sensitivity of transducer coils is very non-linear.

The DC resistance can give you a lot of information about how a transformer will perform, if you know the turns ratio. A HV power transformer with a high DC resistance across the secondary windings will sag more than a transformer with the same turns ratio and a lower indicated secondary DC resistance. A higher DC resistance on the primary of an output transformer with a known winding ratio will typically yield more primary inductance than a unit with lower DC resistance and same turns ratio. I have quite a bit of experience with salvaging transformers of unknown origin for home-brew amps and determining their suitability for various applications. The DC resistance can tell you a lot, but only if you have another piece of data or information that will help you to interpret what the indicated DC resistance means.

[Enzo]

While all that may be true, it is the long way around to get at the fact the wrong reverb pan was installed. The 60 ohm winding tells us from the chart that this was an E pan rather than the A pan required. The transformer impedance wouldn't have changed. And i would never expect the DC resistance of that transformer secondary to be remotely as high as that of the tiny wire in the transducer. I don't expect any sag in this reverb transformer, if nothing else it is a class A circuit, but mainly it puts out no power to speak of anyway.

Did you look at the reverb drive circuit in the Fender amps as a small power amp? It is, you can connect a little speaker to the drive and listen to what is coming out.