Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on January 31, 2015, 06:01:46 pm
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Hi guys, Taking a break form the challenges of old amps :cussing:
After seeing Plexi50 thread on the 6G9 Blonde http://el34world.com/Forum/index.php?topic=17334.msg173562#msg173562 (http://el34world.com/Forum/index.php?topic=17334.msg173562#msg173562) and having a full 18w set in the cupboard, this was a great looking build.
In the throws of getting some parts required, a couple of things popped up................
The PT http://www.classictone.net/40-18035.pdf (http://www.classictone.net/40-18035.pdf) gives me some options on the circuit.
The 120mA rating is enough for 2x EL84 and 5x 12AX7.
An EZ 81 has plenty mAs to do the job.
Q. Is there any good/bad reason to stick with the EZ81 or going to an 8pin type rect?
Trying to stick to the original schematic, it does not have screen resistors on the power tubes and comparing to Dougs stout, 1k resistors used. (a no brainer so they will be added)
There is also no grid resistors on the power tubes and 8.2k seems to be the norm?
There is no coupling cap at the entrance to the PI, again needed?
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Is there any good/bad reason to stick with the EZ81 or going to an 8pin type rect?
EZ81 is fine for a pair of EL84s.
There is also no grid resistors on the power tubes and 8.2k seems to be the norm?
I'd use them.
There is no coupling cap at the entrance to the PI, again needed?
Not needed. Each preamp already has a coupling cap prior to the mixing resistors.
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Thanks Sluckey.
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Hi guys, Another question, can I use one of the MIXING resistors (220k) as my insertion point for the reverb. Thanks
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can I use one of the MIXING resistors (220k) as my insertion point for the reverb.
Probably. But now you will need a coupling cap at the input of the PI.
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Thanks Sluckey, A slow start. I've had to do a lot of :think1: with the layout.
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Hi guys, The next bit is a bit OLD school, an idea that I don't think is new but used by our local amp builders back when.
http://www.ozvalveamps.org/goldentone/suitcaseunder0526rdw.jpg (http://www.ozvalveamps.org/goldentone/suitcaseunder0526rdw.jpg)
The "Valve Tree" was used by a couple of sockets and using the pins was away of mounting components directly to the sockets.
Using a disc of PCB is less bulkie and simple to use and great for tight places.
Another idea is to use bottom mounting sockets, this makes "PREFABING" outside the chassis a easier task and the finished unit can be easily mounted.
The layout sheet shows the component position.
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Timbo, I have never seen this "Valve Tree" PCB. Did you make them? If not, where did you get them?
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Timbo, I have never seen this "Valve Tree" PCB. Did you make them? If not, where did you get them?
They are also called vector sockets. They were popular in compact communications radios. Angela and eBay have this style...
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Thanks guys, Ed, I had to make them just to see if it would work.
It's a costly thing to get made, the design part was going to be about $250
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.
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WOW :huh:
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:dontknow:
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:dontknow: :dontknow:
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:l2:
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:l2: :l2:
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Interesting approach. Sort of a 3D/stand-up version of Merlin's development boards? (http://www.valvewizard.co.uk/developmentboard.html)
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Thanks guys, A bit of progress..................
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Very Cool Job TIMBO :thumbsup:
when you finish that amp (that I consider a wonderfull job), perhaps this will be the next frontier ? :grin:
(http://el34world.com/Forum/index.php?action=dlattach;topic=15611.0;attach=36976;image)
(http://el34world.com/Forum/index.php?action=dlattach;topic=15611.0;attach=36978;image)
Franco
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Hi guys, Last thing to tidy up.
The fender schem has a 27k feedback resistor that is connected to the 8ohm speaker tap.
For convenience it would be easier to connect the feedback resistor to the unused 16ohm tap, so can the same FB resistor be used or does it have to be halved or doubled. Thanks
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Ciao TIMBO
http://el34world.com/Forum/index.php?topic=18132.msg184302#msg184302 (http://el34world.com/Forum/index.php?topic=18132.msg184302#msg184302)
Read Sluckey & DummyLoad
Franco
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Thanks K, I connected the 27k to the 16ohm tap.
This one fired up well. :thumbsup:
I was unable to crank it up, but at low level every thing was working.
The voltages are close the fender schem, although I thought the EL84 plates would be a bit lower as these are 18w transformers.
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O yeah, there was some concern on hum due the proximity of the components, this one is very quiet.
Updated Schematic with voltages.
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Hey Tim, is the centre bolt connected to the local gnd for the valve - as per grounding the central spigot in a valve base? Grounding the spigot in nine-pin novals was certainly a key improvement (and necessity) compared to octal bases. Ciao, Tim
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I like your socket trees. They remind me of some old military gear.
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Hey Tim, I did not ground the bolt, only because I had to etch the boards myself and I just used a marking pen to do the traces, so it was a bit tight to do a trace around the bolt hole. The longest 3mm bolt I can get is 50mm long and at this length they only just fit the depth of the chassis.
I haven't come up with an idea to do a better grounding for each tube, I had thought of a third disc that would have all the grounds of the tubes connected on it, then a solid buss bar between tubes. :think1:
This would mean I would probably need a 75mm bolt, although I can get a 3mm threaded rod and cut it up.
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Top credit for having a go at the tree hanger - I reckon they are great - if only I had some :sad:
The only hanger I've had a chance to use is the 5 eyelet strip that Hammond used to run 5 busbar wires along their organ oscillators tubes - they solder on to the central spigot - a bit cramped, but easy. I like to use the spigot wherever possible, as it makes a great low-noise ground point.
Maybe you'll inspire some Chinese clone company to make them - but that would mean you'd have to sell your versions for $50 :laugh:
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TIMBO, to me your job is great and you've been a really good craftsman
now you have finish and I don't think my idea for a ground connection of the bolt can be applied
the idea is to use nuts and a lug whasher (I hope is the correct name in english)
(http://www.tekkna.it/open2b/var/catalog/b/26344.jpg)
to have a ground connection between the two PCB or on the Top
(http://i.imgur.com/NjycYB7.jpg)
Franco
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Thanks Guys, I think this one has turned out great, so I will investigate some more ideas with the next build I use this method on.
Post a clip ASAP. Thanks
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Hi guys, This one is just about done, needs a few touch up and where good to go.
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Hi guys, I was able to crank it up today and its GRRRRRRRRRRRRRR8.
Lots of good marshally grind but I think it could do with a touch MORE.
I like the crunch Doug gets with the STOUT TMB, so a close look to compare the circuit design.
The only real difference is around the PI and of course the power tube bias.
Is tweaking the PI going change the DRIVE any. Thanks
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It is beautiful and very interesting, thank you for sharing !
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Lots of good marshally grind but I think it could do with a touch MORE.
It seems that this is my mantra of the week, but have you tweaked the feedback resistor or tried the amp without the nfb like 18watt marshalls?
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TIMBO biutiful amp and clean job
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The fender schem has a 27k feedback resistor that is connected to the 8ohm speaker tap.
For convenience it would be easier to connect the feedback resistor to the unused 16ohm tap, so can the same FB resistor be used or does it have to be halved or doubled. Thanks
Just to give an answer for future reference the 27k feedback resistor would need to be increased by multiplying by 1.414 (square root of 2) to yield the same level of feedback. 27k x 1.414 = 38.2k and 39k is the nearest standard resistor value. If you were moving from 4 ohm to 16 ohm you would multiply the resistor value by 2 (square root of 4) and going from 2 to 16 ohm you would multiply by 2.83 (square root of 8).