Screaming Bright Switch Cap??? - 1974 Fender SF Twin Reverb Master Volume Push Pull Switch

[TelRay]

Hello all and Merry Christmas to y'all!!!

I thought I would do one of the many pending good ideas on this long post and revisit Chuck's suggestion to increase the REVERB wet signal (post #350)

1) change the 3.3M resistor to 2.2M
2) Change the capacitor across it to 15P
3) Change the "reverb attenuation" resistor to 180k
4) Change the 470k load resistor off the "reverb attenuation" resistor to 1 Meg
This would leave your dry signal very close to where it is now, but increase reverb/dry signal ratio such that the reverb signal is about 5dB higher than it is now.

I recorded the following video in which I play a chord before and after the MOD. I hear more volume on the reverb and was able to measure about a 3 dB increase (RMS) by selecting the same portion on both audio files (before and after) in which there is only reverb (after I have palm muted the chord).

Thank you Chuck!

[TelRay]

and...

about the other pending issue, the TREMOLO

I have been investigating what should the STANDARD value of the Photoresistor be (LIGHT / DARK), what other users were reporting as NORMAL values and finally what were the values in MINE.

First I took the roach apart to be able to access the Photoresistor and recorded the resistance values in the DARK and LIGHT (5 mm away from a bright LED lamp).



I re-assembled the "roach" to put it back into the amp (i thought i would get some improvement after cleaning the photoresistor's surface... not the case)

Here is a graph with the STANDARD, USER REPORTED and MY measurements.

My conclusion is that, however I am glad that the LIGHT value is so low (400 Ohm seems great to let the full sound go through), the DARK values are waaaayy to low (250K compared to the 5-10 MOhm expected) and this is the reason why the "valley" (no sound) is not deep enugh in my TREMOLO.



I guess I could just order a new "roach" for about $15, but I think I would play with a few different Photoresistors for a fraction of that price and really model the TREMOLO to my taste.

I was wondering what is your take guys on whatever I should try to get very low (500 Ohm or less) DARK VALUES or it should be OK with anything below 2 KOhm.

[pdf64]

Just to note that the light value is the ‘valley’, the dark value is the ‘peak’, ie the normal non attenuated level.
In regard of the trem effect, perhaps more important than the LDRs light and dark values are the fall and rise response times that it takes to reach them. It’s those that create the ‘shape’ (over time) of the modulation’s affect on the signal.

[Mick Bailey]

The DIY device looks to me that it would have too much light leakage to be entirely satisfactory. LDRs need very little stray light to affect operation and they need to be 100% light proof - check where the leads leave the heatshrink.

[Chuck H]

These two ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The rise and fall time will depend on the waveform of the oscillator and the response of the lamp. Though it DOES seem that your tests reveal poor performance of your LDR. Roaches are known to go funny so I don't think it's surprising. You can scope the wave form of the oscillator to see it's shape. I found this very useful on a trem project I did a few years ago. Though mine never looked like a proper sine wave so I don't really know just how pristine it needs to be. Perhaps someone familiar with this trem circuit can report on what the oscillator wave should look like for "good" operation.?. And then I'd just replace the roach.

[Helmholtz]

To measure the dark resistance, the LDR must not see any light at all - and it may take seconds for the resistance to settle.

The performance of a light leaking assembly will improve once the amp chassis is covered resp. installed back in the cabinet.

Neon lamps are very fast reacting. But they probably just switch between off and on, making the voltage across them look squared. I don't expect the wave shape of the oscillator to be critical. The neon light will be pulsating at twice the oscillator frequency.

But LDRs are slow, turn-on time constant is a few ms, turn-off time is considerably longer. So the modulation curve will get smoothed.

[Mick Bailey]

Another factor is the colour spectrum the LDR is most sensitive to. Pairing a yellow/green LDR (for example) with a neon may not give the best results.

[TelRay]

Just to note that the light value is the ‘valley’, the dark value is the ‘peak’, ie the normal non attenuated level.

Thank you!
I guess that my logic was that when the effect is engaged, the sound was going trough the LDR and that's why I thought that HIGH RESISTANCE = DARK = NO SOUND = VALLEY
But I guessed wrong

The DIY device looks to me that it would have too much light leakage to be entirely satisfactory. LDRs need very little stray light to affect operation and they need to be 100% light proof - check where the leads leave the heatshrink.

thx Mike!
Actually the way I re-wrapped the "roach after cleaning the resistor and measuring the values provides a much darker enclosure than the original one (see third photo to the right, where i added insulation tape to close both ends of the cylinder created by the heat shrink)

Though it DOES seem that your tests reveal poor performance of your LDR. Roaches are known to go funny so I don't think it's surprising

yep... I thing that LDR has lived far too many moons

To measure the dark resistance, the LDR must not see any light at all - and it may take seconds for the resistance to settle

Herr Komissar! yes! I did wrap the resistor and hels it with my fingers to measure the DARK values

Another factor is the colour spectrum the LDR is most sensitive to. Pairing a yellow/green LDR (for example) with a neon may not give the best results.

That I did not know, thx! I've seen the neon glows orange/reddish so I guess I should look for an LDR with high sensitivity in the 650-700 nm?

My question remains open. How important would be that the LOW resistance values are below 1K Ohm and the HIGH close to 10M Ohm?

[Helmholtz]

I've seen the neon glows orange/reddish so I guess I should look for an LDR with high sensitivity in the 650-700 nm?

Yes, sensitivity maximum at 650nm is perfect for neons.

[Chuck H]

How important would be that the LOW resistance values are below 1K Ohm and the HIGH close to 10M Ohm?

Well I would say that if you want to be able to dial in a very staccato tremolo with the intensity up full then the low needs to be as low as possible. Well below 1k. If that doesn't interest you much, and it probably doesn't, then I'll just say that 1k as a minimum offers about 24dB signal reduction rather than "off" with the intensity up full. Since you can dial in the intensity with the iNTENSITY control I still think as low as possible makes sense. As to the high resistance, it really doesn't have to be any higher than 1M to preserve signal integrity. But there is likely something about the transitional resistance between dark and light that makes a bigger difference in the smoothness or choppiness between the high and low states. It would seem to me that the higher the resistance the more you would get a 'high-low-high-low' effect. Where a lower resistance might give a more 'high-medium-low-medium-high' effect. But I can't be certain of this. LDR's are cheap enough to merit experimenting I think if you are interested.

[TelRay]

thank you guys!
the only thing is... were to buy them?
My local electronics shop had only 1 type (higher resistance value was way too low, about 5K) and in MOUSER I could only find one (?!) that looks like the one the amp has now. This other one I am not even sure about the resistance values.
I always have the alternative of buying the whole assembly, but I am not sure what I am going to get and it takes away the fun of customizing the effect.
The guys at antique electronic supply do also sell photocells for morley pedals (no specs detailing resistance, though) and it's $2.50 + shipment

[Chuck H]

As they say... If it's not in the specs specifically it's because they don't want you to know about it when choosing!?! Welcome to my world. I'm sorry I don't have experience in this matter though. But it does offer you the opportunity to key into an area that most do not have knowledge of.?. Good or bad? More trouble for sure.

I would think that anyone offering a pre-assembled roach has actually done enough research to sell a useful product. Should it come to that.

[TelRay]

...anyone offering a pre-assembled roach has actually done enough research to sell a useful product. Should it come to that.

true... but, where the hell are they getting the photoresistors from?
In my dreams I enter into a shop, get 100 photoresistors and with a multimeter in hand select 5 with different ranges to experiment with

[Mick Bailey]

On of the fastest and most extreme light/dark resistance LDRs I've come across is part of the Silonex NSL-32SR3 vacrol assembly. However, I can't find the LDR as a standalone part. I use these in compressors and trem pedals as well as some amps. It's possible to run the LED side off a neon supply voltage, though. With LDRs you have to watch the voltage spec in tube amps. Faster devices are sometimes rated too low to be useful. Depends on the circuit and what the LDR sees across it.

[Chuck H]

true... but, where the hell are they getting the photoresistors from?
In my dreams I enter into a shop, get 100 photoresistors and with a multimeter in hand select 5 with different ranges to experiment with

I'm having the same issue with pots right now. I absolutely KNOW what features are available. I absolutely KNOW they can be combined in the arrangement I require. But because I'm not on the inside track for getting a custom combination of features (don't speak any east Asian languages and I don't have the clout of Allparts, WD, Ernie Ball, etc.) I'm having trouble securing a makers process before seeking funds for an entrepreneurial venture. I can't say the quest has been anything short of exasperating. To know and not be able to realize is the hardship of our station. But I'm not planning to give up, though I may need to make some practical concessions initially.

You shouldn't give up either. Learn about LDRs so you can interpret the spec sheets. Then learn who offers specs you're interested in. Then discover if those offerings are available by the each via any electronics distributors. You will likely find that most of what you're interested in needs to be ordered in the thousands to acquire. You will also likely find that whatever IS useful and available is already being used by the people that make replacement roaches now, BUT... You don't know if you don't try. It's likely you would be wasting your time on a quest that's already been discovered to it's best end and from that perspective you should simply allow them their profit and buy a roach. But if you like this sort of thing, and I guess that "I" do a little, the most it will cost you is some time and you will learn some arcane photo resistor knowledge that eludes most of the posters on these forums. Including me. These are your options the way I see it.