DR: OT @ 2 Ohms/ 10" spkr Q.

[Chuck H]

I'll have to admit that I'm a dinosaur. I still use attenuators. Somehow I've never warmed up to power scaling. I just let the amp do what it does, simulate the load and let some of that get to the speaker as needed. Not a bad way to go at all IMHE. I've never built a power scaling amp. Maybe I'd convert if I did. Somehow I just can't imagine a Marshall that typically has 420Vp and all the other amp voltages following suit with current sag, etc. sounding the same if you tank the plate and screen volts and twiddle the bias in "close enough" Just saying. An amp sounds like an amp when you use it like an amp. Whatever technically "works" not withstanding. But I'm also a proponent of doing whatever sounds good. In that light there's plenty about power scaling I don't know. One thing is for sure. It's definitely easier on the tubes than an attenuator.

[Sea Chief]

Chuck- are you using a DIY attenuator? trying my 5F1 maxed (for 2 mins sat a'noon!) the strat got just as much OD as the SG.. so maybe if I built one, using the 5F1 as a guninea pig for the DR: Ive got the extra treb on tap in case that subsided slightly. Strat sounds -much- better than SG in the 5F1 fwiw, SG's nr unuseable with this.

[Chuck H]

Juan's glowing write up on VVR/power scaling aside, I think even HE would agree that for an amp like a 5F1 a simple built in attenuator is an easy and fun way to go. And yes... It's a DIY attenuator I use.

[Sea Chief]

I think thats a good idea Chuck. As its not complicated by all accounts, could you run me through your components.. maybe I could follow your plan if thats ok? would your 5F1 one be useable on a DR too or do they have to be similar wattagey like a 8w rheostat thing for a 5w 5f1, and a 25w type for a 20w DR? (hope to use with the DOM).

[Sea Chief]

Juan's glowing write up on VVR/power scaling aside, I think even HE would agree that for an amp like a 5F1 a simple built in attenuator is an easy and fun way to go. And yes... It's a DIY attenuator I use.

Chuck forgive me: Ive just recalled it was you who outlined for me with a circuit diagram, the DIY attenuator.

I cant for the life of me remember where/ which thread its on tho. found it last ev.

In meantime could you just remind me which in your opinion the better one to buy: Ohmite RJS50R (50 ohm, 50w).. or Ohmite RJS25R (25 Ohm, 25w). Ive got both sourced ready to snap up right now.

[Chuck H]

For the 8 ohm unit you need a 25 ohm rheostat. For the amps you own you need at least a 25W rheostat but a higher wattage rating is fine. So, for your purposes, the RJS25R would be the choice.

[Sea Chief]

Ok Chuck- done that then/ winging its way from US (one you linked to in fact). Spkr in the DOM is actaully 16ohm.. but main 2 amps 5F1, DR.

I''ll get the DR out & re-meaure across a pair of green 6.3 VAC heater for a check, & re-measure plate V.. last time this rose by 10v or so to 440v for some reason from checking before.

Thanks SC

[Mick Bailey]

You can temporarily replace a cap from the component side by clipping the existing cap close to the body and soldering onto the existing leads. When you're happy with the result you can do a permanent replacement next time the board is out. You have to work quickly so as not to melt the original joint, otherwise the clipped lead can fall out.

Interestingly, the VHT 12/20 has power scaling built in and it works very well until oscillation kills the MOSFET due to the absence of a grid stopper. Easily fixed with a 200R resistor.

[Sea Chief]

Hi Mick- sorry Im not with you at all/ have no idea what your referring to (attenuation, the high plate V, power scaling?).

I think I need to refresh the thread; a few avenues have occurred & my arse is down one, my elbow another, and my head floating s'where between!

Im going to concentrate on 2 things. Trying to get plate V down (so amp is safer for..) attenuation.

Thanks SC

[J M Fahey]

Trying to get plate V down (so amp is safer for..) attenuation.

Wonder how you intend to do that .

Specially because 10V relative to 440V is *nothing*.

[olddawg]

I think thats a good idea Chuck. As its not complicated by all accounts, could you run me through your components.. maybe I could follow your plan if thats ok? would your 5F1 one be useable on a DR too or do they have to be similar wattagey like a 8w rheostat thing for a 5w 5f1, and a 25w type for a 20w DR? (hope to use with the DOM).

Ya know... I've done it both ways. I found out that after buying parts, enclosures, etc... Plus my time, it was more economical just to buy the Weber Mini Mass which sounds better and is more versatile. If you are not in the USA, the shipping might be prohibitive. But I would email them anyway, you might be surprised. They are very timely in their responses. What could it hurt?

[Sea Chief]

For the 8 ohm unit you need a 25 ohm rheostat. For the amps you own you need at least a 25W rheostat but a higher wattage rating is fine. So, for your purposes, the RJS25R would be the choice.

Hi ChuckH (been away for a while/ got back to find Ohmite RJS25R on doormat).

Ok the RJS25R here.. measures 29 Ohms between lug 1, or lug 3 and the middle lug, dialing pot it ends at 4.5 Ohms.

Best make sure that seems normal?

gratefully SC

[Sea Chief]

Wonder how you intend to do that .

Specially because 10V relative to 440V is *nothing*.

Ok understood JMF (Ive been away for a bit- thanks for your reply on 7th).. I spoke to a VG amp-maker personally who said 440v on a DR thesedays is nothing out of the ordinary, so I might try & get it down a bit later but for now Im sticking as is.

[Chuck H]

Hi ChuckH (been away for a while/ got back to find Ohmite RJS25R on doormat).

Ok the RJS25R here.. measures 29 Ohms between lug 1, or lug 3 and the middle lug, dialing pot it ends at 4.5 Ohms.

Best make sure that seems normal?

gratefully SC

It's normal. You may get slightly different readings too if you jostle it in the fully rotated position at either end. Those things are pretty clunky, really. There may also be some flux or a harder than typical anti corrosive coating interfering that will wear with time. And because these things are USUALLY meant to attenuate power on big, course equipment, the terminals are made from a non audio grade copper that is tempered for toughness, higher in resistance, more likely to develop a fine oxide layer and pretty much welded rather than soldered. Don't worry about it.

[Sea Chief]

Hi Chuck- it look a heavy duty beastie thats for sure. Ok so following your diagram I saved, I'll get a 1r 10w, a 10r 25w, a DPDT footswitch (is that the type of dpdt) and I guess two 11A Switchcraft mono jacks.

Would I need some heavy duty wire, maybe 1 of the 3 from a mains cable? (always seems a convenient quality wire at hand).

Im not quite visualising how the circuit actually physically would be- in a box? I think you mentioned your on a 'board' or something? do you have a pic you could show me?

Thanks, SC