Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on May 16, 2013, 01:16:44 am
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Hi guy's, Tubenit has again come up with another design and cause it has BLUEZ in the name I just had to build it.
First up, the last build did not go to plan (not that I had a plan)and this left me with a chassis full of holes that I had no idea :think1: what to fill them with.
So comparing what I had and the SWEET BLUEZ schem it was a great fit.
I have a fixed bias 6V6 power tubes ready hooked up,so I don't need to touch them
I had a SS rectifier and was going to leave it as is but cause I removed the filter caps (for smaller value) I decided to put in a socket for a tube rect, I also have a SS plugin that I will try when finished
I removed the circuit board to clear it of the components from the other build and was trying to redesign the layout for that board but it just wouldn't work and having a rethink I decided to do a POINT TO POINT as I already had tag strips between the sockets so I added a few more across the front. This gave me hours of fun to work out :BangHead: and I couldn't find too much on the web on how the experts do it.
I think I have worked it out and have ended up with a few long runs (about 8") so a bit shielded wire to be used
I have pulled apart a couple of old PTP amps and the only thing that stood out was that they grounded any thing that needed to be grounded was made directly to the chassis at any point. In the photos you can see that the tag strips have a tag at either end of the strip that is fixed with a rivet to the chassis and some are near a cathode connection so is this a good idea to use these as my grounding points through out the circuit or would it be better to run ground wires for each tubes assoc. grounds (cathodes, pots, filter cap, etc.) to just one tag (not grounded to the chassis)and daisy chain them together back to the STAR ground :think1:
Thanks :icon_biggrin:
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Timbo,
I look forward to the rest of the story and how this works out for you! I'm thinking it will be a great sounding amp.
FWIW, I had a point to point Dano Centurian 275 yrs ago and it honestly was my favorite of all the production amps that I've owned (Fenders, MusicMan, Carvin, Epiphones, etc....) It had so many ground spots in the amp that it was ridiculous & it was still quieter than the original Princeton Reverb I had. Having said that, I always use Hoffman's grounding scheme and my amps are really quiet at idle.
With respect, Tubenit
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Do a search for John Hynes's Gothic Ring layout for an example of PTP.
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Anyway you can throw this up as a jpeg I need to read up on how to print from jschem. again If Im not using stuff all the time i forget it.
This oldshimers sucks (LOL). I guess I shouldnt have inhaled :l2: :happy1: :happy1: :d2:.
Probably why i love these things so much.
Thanks Bill
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Timbo is doing the SweetBluez FX & Reverb
I think the SweetBluez FX & vibrato could be a great one for blues.
With respect, Tubenit
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Hi guy's, This http://www.guitargear.net.au/discussion/index.php/topic,37080.0.html (http://www.guitargear.net.au/discussion/index.php/topic,37080.0.html) just came in on my Aussie forum. Some good pics on how it was done here ( I don't think it was done much different to fenders) again using the chassis ground points where convenient. I really like the old school stuff but is it just asking for TROUBLE :think1:
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Hi guys, Great news, Sweet Bluez is pretty sweet. I can dialin some really nice tube driven sounds.
I do think that some tweaking is needed, only cause I can get it with the trim on 1 anything above that is UGLY.
The reverb did not work in the original position, so moving the mix resistor to the new position works but induces hum with level at max but at about 8 its fine but not much verb happening :think1:
I was wondering if the .01 and .047 either side the MIX resistor is required :dontknow: not that I think it is causing the hum
As for the hum there is very little. :icon_biggrin:
Voltages - Sheet 3
It still has some FIZZ with nothing plugged in so I am thinking that it might be the PT after all it is 59years old.Thanks
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Just a couple of little changes :icon_biggrin:
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PER your voltages chart ........... you are using all 12AX7's ???? Is that correct? I don't think those gain values will work? And as an FYI, I find a 12AX7 in this type of FX stage to be unuseable for the tone I want.
Dropping down to 12AY7 or 12AV7 value in V1 will be a huge help. And I would move the 12AU7 to the FX stage.
Here are some things to consider ........................
My D'Mars ODS has a 500k volume pot, FYI. You can parallel a resistor across the volume and trim pot to achieve the suggested values. If those work, maybe substitute a different pot if you don't like where the "dial" is.
Maybe use the 4b triode simply as an OD boost? You may not need that triode most of the time because of the boost in the FX stages?
With respect, Tubenit
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I was wondering if the .01 and .047 either side the MIX resistor is required
Both caps are necessary. The .01 prevents V4b plate voltage from reaching the reverb pot. And the .047 prevents V3a grid bias voltage from reaching the reverb pot, and it also prevents the reverb pot from interferring with the bootstrap bias of V3a.
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Timbo,
Just as a reference point for gain levels, note that my TBM preamp & FX is somewhat similar to the SweetBlues design. And in fact, I have two gain stages prior to the FX.
My gain is pretty smooth and can still get a reasonably clean headroom with the OD engaged. Note that I am using 12AY7 and 12AV7 tubes in the preamp and FX.
Here is a soundclip demo with volume around "5" which is where I usually play it:
http://www.soundclick.com/player/single_player.cfm?songid=11968741&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=11968741&q=hi&newref=1)
With respect, Tubenit
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Hi guys, I'm sad to say that Sweet Bluez is just not happening :sad2: I was able to get some nice sounds but this was only on certain settings. By that I mean if you try to get the sound to a higher volume the fizz and buzz (on the decaying notes was annoying :BangHead:) and no matter what I tried I could not SMOOTH it out to an appealing sound.
The buzz and fizz is the same as in the previous builds, so this has put the breaks on this build for the moment.
Tubenit, thanks for putting your thoughts on paper. :icon_biggrin:
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Timbo,
Thanks for giving it a try and posting the results! Sorry that it did not work out for you.
With respect, Tubenit
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Timbo I must ask, you keep having the buzz and hiss problem you have tried some very classic builds that usually sound great. Are you using the same chassis and transformers for each build. try one with new chassis and transformers , maybe your transformers are bad maybe your heaters wires are not twisted well, there are a lot of things that could be the problem.
Sometimes starting fresh works, Im just throwing that out there because you have done alot of work and have been frustrated at every turn. I guess I just wish i could help.
In the future I want to try this design also.
Bill
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Hey Bill, I used the same PT , OT and power amp section for the Hiwatt builds and for the Sweet Bluez and all produced the same buzz and hiss, so I was thinking that it was the old PT.
It now resides in the " it almost worked cupboard" :l2:
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I have one of those shelves too my friend. My wife says Im a horder (LOL) but I can always go rob a part when i need one.
Bill