Right back to basics/ pg1 Qs.

[DRH1958]

[QUOTE=Sea Chief;390061

Getting in ad replcacing the grids is a major exercise. Why arent they 1k as they are, to be more sturdy anyway?



The reason for the 470 ohm value I'm afraid is just that's the way they did it on the original. It's a re-issue,so....

[DRH1958]

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Re the xfmr query: you see Im trying to evaluate whether in your opinions on here, I have indeed been either done over by way of an odd xfmr pushed my unsuspecting way (Id have thought theyre tested at factory for their HV, but remember the spiel always says +- 10% or something so if twds top of this range but just 'in' still. And, theyre made a generic xfmr & not specifically for DR's afaict IE "for a DR" added by the Co I bought from) or have to accept either they are generally much higher V (in which case would I not be Querying the Co's "for a DR" info) or as said if within the +-10% range then some will just be higher: the lower ones he likely uses for his pro builds.

All I'd do without the knowledge you guys have, is call just for him to spiel the +-10% tolerance thing, or "thats easy just put in a 5Y3", or it is "a bit of a lucky dip", or "its perfectly normal SF DR's run a high ~ 430v with a 5U4" (thing here tho is all builders do the BF circuit with the 5AR4 by default unless they specify an xfmr for an SF, surely).

Im wondering if I should ditch this PT you see, and buy the chinese (gasp) standard RI. A waste of nr £100 (need a 5AR4 as I sold orig one as had no need) + the work, but if it means using with a attenuator Im substantially safer.. then surely the only way to proceed.

I guess the only real leg you have to stand on is if the tranny is putting out higher than the 330-0-330 VAC that is spec'd. If that measure too high, check your wall mains to just make sure so you can tell them "hey, I've got exactly 240 v out of the wall".
I did find an original AB 763 Deluxe Reverb schema and it is marked as 330-0-330 on the secondary of the PT, so they did spec the correct one. It comes down to if your particular one is faulty. Measure your AC secondary voltages. Do all three, 5v, 6.3v and 330v.

[Chuck H]

Chuck is this one correct type? RJS50R Ohmite | Mouser

The rheostat should be 25 ohms. That one is 50 ohms.

Ohmite rheostats aren't arranged as "25R/25W" "50R/50W" "100R/100W" They are available in many non analogous values and wattages. Perhaps you are only seeing them this way because of the limitations of common search engine criteria or simply that those would be popular values.

The rheostat you use should be valued at 25 ohms. It can be rated for any wattage at or above 25 watts.

[olddawg]

The mini mass has a reactive load, an actual speaker motor in it. They used to sell the speaker motors for DIY builders but I don't think they do anymore. If you order one, consider the foot switch option. Truthfully, you couldn't build a mini mass yourself for what they charge. And they are built like a tank. You said you didn't like the Epi VJ amp. It's basically a Champ type amp. 2 tubes (12AX7 and EL84). I was driving one with a compressor and and a OD pedal dimed and had to use an attenuator with it many times for smallish venues. Mine is a head and I was using a 2x12" cab though. Not much clean overhead but it really screamed. Recorded with it too. The engineer even said that little f@@ker is loud! I've changed bands and use A DR clone with two footswitchable channels and a master volume now. It's more versatile and I have haven't needed the compressor. It should be said that a lot depends on how and what you are playing. In the end, tone is in the hands.

[olddawg]

I would recommend the 50 watt version, especially if you are attenuating a lot. Btw... You can use one a as a load and they have a line out. In practice, I have found that attenuators work best when used to take the "blaring" edge off of an amp. Over-attenuation really affects tone. Of course, as always, it's subjective. Your mileage may vary. Many people are happy making a resistive load attenuator with an L-pad mounted in a coffee can.

[g1]

As far as the high voltage from the PT, Mick Bailey had something strange happen with the same model PT. Maybe he'll remember and comment whether anything similar could be possible:
http://music-electronics-forum.com/t31610/

[Dave H]

Im wondering if I should ditch this PT you see, and buy the chinese (gasp) standard RI. A waste of nr £100 (need a 5AR4 as I sold orig one as had no need) + the work, but if it means using with a attenuator Im substantially safer.. then surely the only way to proceed.

Don't ditch the PT. You could easily reduce the voltage from 440V to 410V by using the traditional method of putting a resistor in series with each plate of the rectifier tube.

[Dave H]

My Champ on the other hand IS a bit of a disaster: little od even maxed

It's a 5F1 isn't it? That's a low gain circuit. Don't give up on it. They can sound good through a 12" speaker in a decent sized cabinet. To get it to od disconnect the 22k feedback resistor and add a cap across one or both cathode resistors of the 12AX7. Use a smallish (0.68u) value to reduce the excessive bass. Try it with your attenuator. You are more likely to get good results attenuating from 3W to 1W than from 22W to 1W.

I have a homebrew 'Dominator Lite' (just the non trem channel) and a VJ head. After doing a few simple mods the VJ sounds almost the same as the Dom through the same 12" speaker (V30). I used to gig with the Dom cranked up but now I'm only using the VJ in the bedroom (and it's still too loud).

[Mick Bailey]

As far as the high voltage from the PT, Mick Bailey had something strange happen with the same model PT. Maybe he'll remember and comment whether anything similar could be possible:
http://music-electronics-forum.com/t31610/

I remember now! That now seems like a very long time ago. Since then I've fitted quite a few Hammond PTs. That particular instance was a specific fault, but the output voltage has certainly been higher than expected in pretty much every case since. This leads me to think that the primaries are being wound for maybe 230v rather than 240v. There's a lot of confusion with European voltages. The UK is on 240v, but my own voltage regulalrly exceeds this - usually runs at 247 or more. The European Harmonised Standard is 230v and the tolerance is wide enough to include 240v so we're not left out. Almost every manufacturer sets their primaries as either dual 115v, or single 230v.

Now, let's say the voltage is actually 245v running into a primary designed for 230v. By the time this is increased by the turns ratio and rectified it results in a much larger increase in secondary voltage - often 30v or more.

Actually, a few months ago I repaced a Vibrolux transformer with a Hammond version (291CEX) and had to tame the B+ with a MOSFET to drop 36v.

I also get my transformers from Bluebell - Philip is one of the good guys and I've been buying Hammonds from him for more than 10 years and I've always found him to be a really helpful, genuine guy. Maybe an email directly to Hammond may shed some light on the higher-than-expected voltage. I've just put it down to EU standards but there may be more to it.

[Dave H]

The UK is on 240v, but my own voltage regulalrly exceeds this - usually runs at 247 or more.

It must be the new British standard I often measure nearer to 250V now. In the good old days it was a always a rock solid 240V. You could have calibrated your (analogue) meter to it.

Edit:
But it's still inside the EU spec. at 250V! 230 + 23 = 256

[Sea Chief]

Hi Mick- thats interesting you find them all higher than expected. The 291 is meant to be an upgraded 290 with repsect to the 230v (290) and 240v (291) so I dont know whats going on. Ive read a few www stuff about the 291 high 2ndary voltages.

I emailed hammond today, just had reply with "well the AA172 circuit, only one I see available for public viewing, has 440v plate voltages exactly like yours." So Im replyng with the AB763 schematic as the only DR circuit used RI or kits etc. They should know this being perhaps ~the most famous amp ever~.

No I dont think Bluebell sold me a one-off esp high then, I doubt it as Ive bought xfmrs form Philip before and a more affable guy I doubt there is out there. I think your turns ratio idea makes sense then, so the prob lies maybe with the primries not being up to 240v spec.. whch seems odd as its a newer version specifically with only one 240v primary set.

[Sea Chief]

It must be the new British standard I often measure nearer to 250V now. In the good old days it was a always a rock solid 240V. You could have calibrated your (analogue) meter to it.

Edit:
But it's still inside the EU spec. at 250V! 230 + 23 = 256

But if such unusually UK high main voltages are responsible, a standard original BF (330-0-330 with plate v @ 410v) would -also- be a whopping 480v with an 5AR4.. but none ever known to be so excessive. Or an RI would be showing not 396v as the schematic shows (I wonder why its not also 410v if its such a perfect redo?) but summat like 450v.. but A) that surely wouldnt be acceptable to fender, and B) no-one out there mentions seeing excessively high RI plate v in UK.

So it has to be the PT, surely.

[Mick Bailey]

The 291 is meant to be an upgraded 290 with repsect to the 230v (290) and 240v (291)

The 290 and 291 are differentiated by the lamination alloy - the primaries are the same voltage for both. the 1 (rather than 0) denotes a 29M6 low-loss alloy.

Now, I used to buy 'regular' Hammonds, but I wonder if the higher spec results in higher output voltage due to the lower losses? In recent years I've only been buying the high spec versions.

[Dave H]

It could be a combination of things. For example - High mains voltage, PT wound for 230V not 240V, Hammond PT having better regulation than the original, a low bias current setting etc. Have you considered adding resistors in series with the plates of the rectifier tube as I suggested above?

[Sea Chief]

It could be a combination of things. For example - High mains voltage, PT wound for 230V not 240V, Hammond PT having better regulation than the original, a low bias current setting etc. Have you considered adding resistors in series with the plates of the rectifier tube as I suggested above?

No I havent prized it apart again yet. Im sure tho when I did a thread on firing it up (Final checks: DR fire-up or summat) and found the huge voltages still after advised to 1st get a 5U4 in there, Id have been informed of this idea at the time tho? I was scratching about trying and trying to get the damn V down. I'll have to view thread again & see if I tried it.

There's always a caveat to this idea tho surely Dave-?