10W Robben Ford mod questions...

[Bob Simpson]

Should the resistor be one of the white cement/sand ones, or would the finned aluminum-bolt-to-the-chassis type work as well? Is there a difference in the heat dissipation potential?

I've seen 330R and 390R's used and most of the schematics are marked
300-500R. Should I just get a bunch and try them in and out, or is there
some rule of thumb as to value.

How about placement in the chassis. I'm thinking a terminal strip between the On/Off/On DPDT standby switch and PT end bell would work.

Should the first set of filter caps be before the standby switch, and charged, or after?

I read a suggestion to have the choke/resistor hooked to B+ first, and the appropriate output to the switch's middle lug hooked directly to the OT ct, and B+n filter caps...

I've seen pictures/schematics of builds with each type.
Just wondering if there is a "best" way, or if it really matters at all, I guess.

Bob

[Normster]

Should the resistor be one of the white cement/sand ones, or would the finned aluminum-bolt-to-the-chassis type work as well? Is there a difference in the heat dissipation potential?

I've seen 330R and 390R's used and most of the schematics are marked
300-500R. Should I just get a bunch and try them in and out, or is there
some rule of thumb as to value.

Scott has always recommended 330R for 100watt amps and possibly something smaller (220R?) for 50 watt amps. Moss and I settled on 500R for the D'Lite because we wanted a little extra "give" as well as slightly lower screen voltage.

How about placement in the chassis. I'm thinking a terminal strip between the On/Off/On DPDT standby switch and PT end bell would work.

The resistor doesn't get as hot as you might think. Wherever you can find room should be fine provided you pay attention to HT routing.

Should the first set of filter caps be before the standby switch, and charged, or after?

I read a suggestion to have the choke/resistor hooked to B+ first, and the appropriate output to the switch's middle lug hooked directly to the OT ct, and B+n filter caps...

I've seen pictures/schematics of builds with each type.
Just wondering if there is a "best" way, or if it really matters at all, I guess.

Bob

I think most have agreed that the standby should be after the B+1 caps. I don't know what the "technical" pros and cons are, but these are my observations:

With the caps after the standby, you'll get a soft "whump" sound when switching to Play. Also, the caps bleed down faster when you switch the amp off. (No big deal. Caps will bleed down anyway and you can turn the amp off with the Standby switch in Play position to bleed down faster.)

With the standby switch after the B+1 caps, it's very quiet when switching to Play, but pops when switching back to standby. Easy enough to fix by adding a 1KV .001uF ceramic cap to the standby switch.

If you look at early Fender and Marshall schematics, you'll find that it's been done just about every way imaginable.

[Structo]

Bob if this is for your Blue Monkey, isn't there a spot on the filter board for a 10 watt resistor?

I use a 330R by the way.

[Bob Simpson]

I don't think so, Tom.
It's set up for a choke, and a resistor to the reverb transformer.
Just something I'm thinking about.
Lots of possibilities before I start construction.

Bob

[Structo]

I don't think so, Tom.
It's set up for a choke, and a resistor to the reverb transformer.
Just something I'm thinking about.
Lots of possibilities before I start construction.

Bob

Oh, OK, I didn't realize that the BM was set up for a choke.


I would put it on a switch, so you can either have the choke or resistor.

The other thing that you will probably have to do is adjust the dropping string for B+.

[Bob Simpson]

I'm waiting for the NKK On/Off/On DPDT switch from mouser.

That's why all the questions...

I'm thinking up for choke, standby middle, down for resistor.

Any thoughts re the type/composition of the resistor, or will it matter in this instance?

Should be an easy mod if I plan for it from the beginning...

Norm, could you shine a little light on what you meant by the HT routing line?

"Wherever you can find room should be fine provided you pay attention to HT routing."

HT is something I definitely pay attention to... did you mean for shorting problems, or noise problems?


Bob

[Aurora]

Just one point to remember...
The power rating for the "aluminium-bolt-to the-chassis" thingies are only valid when they ARE bolted to a chassis.
Cement type wire wounds requires some air around if used up to their spec's

[Structo]

Yeah, I would use a Cermet 10 watt.

You know, the sand cast white ones.

Unless you do have someplace you could use the aluminum power resistor and as mentioned it must be chassis mounted for heat dissipation.


BTW, do you have any progress shots?

I want to see that blue chassis in all it's glory!

[Normster]

HT routing - just keep the resistor and wiring at the power supply end of the chassis and you should be fine. The d'lite chassis is a little tight but you should be able to mount an aluminum style between the front panel switches and the transformer.