diytube.com

Thanks. I'll try and give this one a shot. Too many going right now. But its a beaut. Thanks again Len

Don,
Nice job.
I would suggest one minor addition - a 5 cent resistor.
Add a 100K resistor from the wiper of the bias set pot to the top of the pot.

Its just a safety thing - if the pot fails, or the wiper becomes disconnected, which they are prone to do, then the output tube bias defaults to full negative (minimum output tube current) rather than to 0V (full output tube current).

Cheers,
Ian

Hmmm, a good suggestion, a little hard to implement the way things are now. I'll look at it tho...

Don

Don, have you tweaked your design since building it and having people gig on it for a few months? I'm thinking of building one.

[Funny thing about the Price of Hammond Transformers...they're made in Canada, but I can get them 15% cheaper from the US, not counting the currency shift in us Canucks' favour!]

Chris

No, I have not tweaked it since I built it. I have two mods in mind tho, one is mentioned here, a fail safe for the bias circuit and the other is to replace the cap and two resistors on the input with a single 10 meg. Some of the acoustic bass players are telling me the bottom end is not what it could be. Piezos like as much input impedance as possible. Actually, make that three mods. The standby switch has to go, go, go.

Reviews for the amp have been very positive, I have had people I don't even know comin up to me to give kudos on a nice amp. I've also had several requests for more but it's a slow process and when you tell someone the price, well, that weeds out 95% of them. If a Fender Blues Junior is $400 why should they pay $2000 for a Hammertone?

I've also said elsewhere that I'm completely unimpressed with the stability of the Sovtek KT88s, next time it will be JJ.

The amp has been in continuous use since May of this year without a singe hiccup (knock on wood) apart from having to rebias the KT88s a couple of times.

Don

dhuebert wrote:...the other [mod] is to replace the cap and two resistors on the input with a single 10 meg. Some of the acoustic bass players are telling me the bottom end is not what it could be. Piezos like as much input impedance as possible.

Absolutely true! Although all gigging upright players should bring a little batt-powered high-impedance buffer or preamp to gigs, coupling directly with that first tube stage is where it's at.

Perhaps an input impedance selector switch for your case, when the amp has to handle most any type of input (ultra hi-z, high-z and lo-z (active))?

Thanks for the reply, and especially, thanks for sharing the schematics and photos.

Chris

dhuebert wrote:The standby switch has to go, go, go.

Care to elaborate?

Jim

Wellll, it's like this: Musicians expect a standby switch, if they don't find one they turn the amp off between sets. Tubes are like cars in that 90% of the wear happens during warm up. I thought it would prolong the life of the tubes to put in a standby switch and give the musicians something they were familiar with. I am not a believer in standby switches and never generally put one on an amp but since this one was going to be used by alot of different bassists, I thought it was a good idea. In the first month of operation I had to rebias twice due to glowing plates, also every time that switch was turned on the KT88s made nasty pinging and popping noises, I was not happy. I decided to disable the switch and educate the bassists to leave the amp on between sets, I am much happier.

http://www.highandlonesomeclub.ca/

Don

Then there is the "sneaky way" just wire in a standby switch that does absolutely nothing except turn on a red pilot light.

Sometimes a placebo works better than the real thing...

Makes 'em feel all warm and fuzzy

/ed B in NH

Listen: I seriously considered that and may just do it yet. But what I'm thinking of is to short the volume control to ground. If I can do it without introducing more buzz into the amp...

Don

Peavey had an interesting standby circuit in their Classic 50 and 100 series amps that used massive numbers of 6BQ5s in the output. They switched off the B+ to everything except the plates of the output tubes. My former partner and I had trouble playing the Devil's Advocate with each other as to why they did it; the only rationale we could come up with is that the output tubes, with the screens in limbo (not grounded, but not at regular potential, either) could conduct just enough to prevent cathode baking. I have an email in to their engineering people to see if they can shed some light on the thought process involved.

A few years ago, I had bought a Peavey Classic 50/50 at an auction for $100. This particular amp had been owned and used by Aerosmith, and was donated for the cause.

It had 4 12AX7s and 8 6BQ5s (all Sovtek). The 6BQ5s were pretty much shot, but the 12AX7s still tested OK, I re-tubed it, and cleaned out all the accumulated dust, hairballs, and crud (and who knows what other kind of residue) inside the unit.

Seeing as how "flat" those 8 Sovtek 6BQ5s were, maybe that particular standby scheme wasn't quite so good after all...

The amp could be run as two 50 watt channels or bridged for 100W.

The amp eventually went to a local area youth church group, and they all enjoy using it, and its little bit of "fame".

I still have the dozen "Aerosmith" tubes (I never throw anything out)... Yellow_Light_Colorz_PDT_06

/ed B in NH

I was also thinking of using a multi-pole switch to kill the audio (as well as control the "idiot light" ), by grounding it downstream from the input VA stages, and before the PI, but then I figured it would just introduce unwanted hum, noise, etc. I suppose that a relay could be used, but then that's just adding another level of complexity..


/ed B in NH

i have been asking around some sites for advise on a first time amp build.

things have changed a bit for me and now my ex-partner (an AF trained radar repair/operator) is willing to do the build for some parts i have if he has a good schematic.

i really believe that in my position (having to buy almost the whole sum of parts) that i should go for as much amp as i can afford. i have a nice chassis and a few parts already but having to buy most all of the remainder, it is apparent that it will not cost me much more to build a 100/200 watter than a 50 watt amp.

this is a request to use your schematic from your BFA.

thanks for your consideration.

cw

Like the library, the schematics for the BFA are "Free to All". I'm kinda pumped to see someone else build and tell me what they think. So go ahead. The only advice I would give is to examine carefully the photo of the underside of the chassis with respect to laying out the preamplifier part of the amp. It is very quiet the way it is, it would be a shame to have someone build it and be unhappy because it buzzed too much.

Don