3 Tech Q's

[Structo]

Hi Henry,

On the Yamaha schematic, what are the devices labeled as 52, 53, 54, etc.?

[heisthl]

Hi Henry,

On the Yamaha schematic, what are the devices labeled as 52, 53, 54, etc.?

Those are encapsulated LED/Photo resistor pairs (>100 ohms light on, <10M light off) that were popular for awhile as relay replacements. They are kind of a pain to use because it's not really a switching device, instead you use them to ground one signal path and at the same time use another one to let an alternate signal pass through. In fact in this amp it took 6 of them to switch from clean to dirty. The thinking was you eliminated relay "pop" with this type of device and also took the possible mechanical failure of a relay contact out of the design. In reality they can actually be less reliable because the photo resistor can drift, Ever have a bad photocell in your Morley volume pedal? In the '80s that was a common warranty repair.

[wjdunham]

Thanks Henry, I'm going to give that circuit a try this afternoon, simple is good

Bill

[wjdunham]

Still having some trouble with this - relay voltage is dropping to around 1.8V when I switch the first relay, the second one doesn't have enough voltage to fire. The Hammond 290FX transformer is rated at 5.5A of filament current, 3 x 12AX7 @ 300mA each is 1A, the two 6L6 @ 450 each is 1A, so I should have plenty for the relays. Should only be pulling 30ma for each relay, where would the voltage drop occur? In fact, when I look at the heater waveform on a scope, the nice symmetrical 5V PP wave gets clipped and pulled down when I ground the relay (looking right at the input to the rectifier diode). Could this be due to the fact that I don't have a center tap heater setup and have the 120R ground reference resistors in stead?

[ChrisM]

Still having some trouble with this - relay voltage is dropping to around 1.8V when I switch the first relay, the second one doesn't have enough voltage to fire. The Hammond 290FX transformer is rated at 5.5A of filament current, 3 x 12AX7 @ 300mA each is 1A, the two 6L6 @ 450 each is 1A, so I should have plenty for the relays. Should only be pulling 30ma for each relay, where would the voltage drop occur? In fact, when I look at the heater waveform on a scope, the nice symmetrical 5V PP wave gets clipped and pulled down when I ground the relay (looking right at the input to the rectifier diode). Could this be due to the fact that I don't have a center tap heater setup and have the 120R ground reference resistors in stead?

3 12AX7 is 0.9A.
6l6 draw 0.9A each, so two is 1.8.

That brings your total heater current to 2.7A.

Are tapping off a portion of the heaters are rectifying it?

[heisthl]

Still having some trouble with this - relay voltage is dropping to around 1.8V when I switch the first relay, the second one doesn't have enough voltage to fire. The Hammond 290FX transformer is rated at 5.5A of filament current, 3 x 12AX7 @ 300mA each is 1A, the two 6L6 @ 450 each is 1A, so I should have plenty for the relays. Should only be pulling 30ma for each relay, where would the voltage drop occur? In fact, when I look at the heater waveform on a scope, the nice symmetrical 5V PP wave gets clipped and pulled down when I ground the relay (looking right at the input to the rectifier diode). Could this be due to the fact that I don't have a center tap heater setup and have the 120R ground reference resistors in stead?

You need the C.T. , without it you will have to use a single diode 1/2 wave design with one of the heater legs as an isolated ground, may still be enough to run a pair of 5 volt relays because most brands will pick as low as 3.3 volts.

[Structo]

On my D'Lite I use the 653-G5V-2-H1-DC5 (Mouser #) relays.
They are the sensitive low signal ones that can latch at lower voltages.
It may be your relays that aren't right.

http://www.mouser.com/ProductDetail/Omr ... hwGQ%3d%3d

[wjdunham]

Tom, Those are the ones I'm using.
I don't quite understand the "using one of the heater legs as an isolated ground". Is this more than just using one of the heater taps into a half-wave rectifier? This is what happens when you let digital guys play with analog stuff

[heisthl]

Tom, Those are the ones I'm using.
I don't quite understand the "using one of the heater legs as an isolated ground". Is this more than just using one of the heater taps into a half-wave rectifier? This is what happens when you let digital guys play with analog stuff

With the 1/2 wave you need to use one heater leg for the diode and the other one for the ground. That ground is only for the smoothing cap and the relays, thus "isolated". You cannot use the chassis ground because it is where your artificial center tap (C.T.) is grounded and you'll have the same problem as before. Next time get a transformer with the heater C.T.

[Structo]

This may help.

http://en.wikipedia.org/wiki/Rectifier# ... tification

[wjdunham]

OK, thanks. Working much better now! Thanks for that reference Tom, I should definitely read that one. Looks like we all need a life here, sniffing solder fumes on a Saturday night.

[talbany]

Yeah an extra 5 or 6.2 VAC winding, connected to a FWB with a larger filter cap (1000uF or my favorite of 2200uF@10volts) for the relay supply referenced to the same ground lug as the transformer C.T. has only 2 parts, no noise, takes up very little space and is extremely reliable for running 5 volt relays. A branch off to a 500 ohm resistor/LED pair can run the pilot LED too. It could be argued that the low parts count makes this method more reliable than a regulator setup. The highest parts count is the regulated voltage doubler method. Besides the extra Radio Shack transformer is a PITA to mount and connect. Smile

If you want to run relays off the existing heaters look at the attached Yamaha/Soldano T50 schematic excerpt - this method works pretty good and you only need 2 diodes, 1 resistor(works as a fuse) and 1 capacitor.

I was in one of Mikes amps not long ago and noticed he runs his LDR's off the filament supply..your right cheap...We looked into that..Plenty of cheap builders doing that nothing new.. They are very quiet being opto switchers no contact noise and no need for the 22m pop resistors...Just haven't gotten around to modifying our boards and still have a ton of the Axiom's..

Tony VVT

[heisthl]

I was in one of Mikes amps not long ago and noticed he runs his LDR's off the filament supply..your right cheap...We looked into that..Plenty of cheap builders doing that nothing new.. They are very quiet being opto switchers no contact noise and no need for the 22m pop resistors...Just haven't gotten around to modifying our boards and still have a ton of the Axiom's..

Tony VVT

If you look at this schematic again you'll see the Yamaha had one relay to decide which bank of opto's got activated.

[talbany]

If you look at this schematic again you'll see the Yamaha had one relay to decide which bank of opto's got activated.

Yeah 3 for the clean and 3 for the OD...

Is that a relay? looks to me like it's just a switch maybe on the front?.. (Sorry if i am missing something got up real early)

do you have the rest of it?

The one I was in was I believe the hot rod +

A friend of mine has one of those Yamaha's (cant remember the model)
They are high insane gain machines..well designed well built amp..


Tony

[talbany]

Oh Yeah Yeah I see it now Footswitch drives the relay,, Gotcha ...Pretty Cool!! Long day!!

Here you go..Ultra fast Vactrols.. Make sure your dark resistance is high enough or it will bleed


Tony