Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Banjan73 on April 30, 2026, 12:57:34 am
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Hello!
I`ve just tested my new home grown amp. The quest this time was to build a bass amp for my son. I googled a little bit, and the Ampeg SB-12 stood out as a nice candidate for beeing a base/idea to my build. My approach for new builds is to be inspired by classic amps, get the schematics and draw them all over again the way I want them to be.
So this amp is inspired by the 6L6 version of the SB-12. I have (as always) used SS rectifier for HV and the filament is also rectified. The tonestack (James/Baxandall) is almost identical, with minor modifications. Power transformer is from a Tandberg Huldra 4 radio, and output tranny is a Hammond 1650H.
Well. The first test is performed with a band. I tested it with my Les Paul, and the bass player tested it with a double bass and an electric bass. I must honestly say that I liked the bass sound better than plugging in a guitar to this one. But thats the point with this one, to be a bass amp, hehe.
I need to do some minor mods/measurements to this one, and I need to build a enclosure for it. I didn`t have two ECC83 tubes, so the second one right now is a ECC81. But tubes are ordered. I suspect the sound to be a little different with a ECC83 in the second position.
Anyway, are there any recommendations for a loudspeaker for this one? I am looking for a speaker with high sensitivity. My first chioice right now is a Mojotone greyhound 12" (101db/W), but I don`t know that speaker. Any other suggestions?
Heres some pictures and schematics:
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Ouch!
I just spotted a minor, but important error on hte schematics. B+ is connected earlier in the filter chain.
Heres updated schematics:
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No speaker thoughts here? Jensen C12N is supposed to be the original, but 97-98db...
Anything els than mojotone greyhound that I should consider?
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Hello!
Some huge updates here.
I finished the amp, and it sounded GREAT on guitar! But.... this was supposed to be a bassamp, and it didn`t live up to my expectations. It pulled out about 15W clean power. In my opinion, it was too little punch and too low wattage.
So... Then I went totally bananas.
The new thing now is to use full bridge rectifier for B+ to produce more voltage for the anode. Output now is about 760VDC (!!!) So, thats a little high for those 6L6 tubes. Well... I had stored somewhere a matched quad of JJ EL34 tubes, and it was time to put the gloves on.
As I think that pictures tells more than my blah-blah, heres the new schematics.
P.S. As my output transformer is rated for 40W, I need to restrict this amp a little bit. Therefore I drop the voltage to the screen with that zener diode to obtain, hopefully, around 40-45 watt...
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BTW. The change of v3/v4 from 12AX7 to 12AT7 is due to that the PI gets overdriven. The schematics for 12AT7 is not updated yet (old values from 12AX7).
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Are there any considerations I should take here? Anything special I haven`t thought about? It`s much more scary working with 700+ volts than 350V, hehe :-)
Please shout out if you guys notice anything.
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R17 and R18 seem to be unnecessarily low, which is why the PI gets loaded and over-driven easily and may not produce enough signal to drive the output tubes. My old Philips datasheet shows Rg1 500kohm max for a fixed bias EL34.
The same data lists 425V B+ for anodes and screens, with 1kohm screen resistors, Raa (the impedance of the OT loaded with a speaker) 3.4 kohm. While it is not super critical, we do not know whether your OT has an impedance in this range. 760V B+ is rather for an 80W amp, but with Raa=11kohm and B+2 (for screens) 400V. In your case, 300V seems not enough even for a 40W amp. R23 and R24 of 2.2 kohm make the situation even worse.
Hope this helps a little
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R17 and R18 seem to be unnecessarily low, which is why the PI gets loaded and over-driven easily and may not produce enough signal to drive the output tubes. My old Philips datasheet shows Rg1 500kohm max for a fixed bias EL34.
The same data lists 425V B+ for anodes and screens, with 1kohm screen resistors, Raa (the impedance of the OT loaded with a speaker) 3.4 kohm. While it is not super critical, we do not know whether your OT has an impedance in this range. 760V B+ is rather for an 80W amp, but with Raa=11kohm and B+2 (for screens) 400V. In your case, 300V seems not enough even for a 40W amp. R23 and R24 of 2.2 kohm make the situation even worse.
Hope this helps a little
Hi.
Yes. This amplifier was orinially designed for 6L6s. Thats why R17 an 18 have that value. I haven`t changed them when I changed the tubes to EL34s. I will take a look at that one. I will also change the PI values due to that I now have decided to use ECC81s there. Hopefully I will then get full voltage swing without too much clipping.
My OT is a Hammond 1650H with a primary of 6.6k.
As this is supposed to be a bass amplifier, I now consider to remove that zener diode because I want as much "punch" as possible. But then I am a little worried that I get too much wattage for my OT. I believe the last voltage measurements are: Anode=770V, Screen (before the zener)=360V after zener = 300V. I am going to run a bit cold bias to restrict this amp a little. So bias will be dialed to 20-24mA.
So you don`t think I need those screen resistors? I am thinking that without them, I will get som red-screening or red plating? I wish to limit the current a little bit. Perhaps 1.5k is better?
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Hello again!
Yesterday, I made some changes according to jansch input and other stuff.
I have got rid of the zener diode, increased grid leak resistors and grid stoppers for the EL34 tube and I have adjusted the values for the ECC81 tube. The amplifier is not tested quite yet, just a fast voltage check different places. Bias is adjusted to 22mA. Voltages is updated at the schematics.
Today I will put on a signal and scope a little bit. I will also check the output wattage of this guy, even though I think my effects resistor (dummy speaker) will get quite hot (its rated 150W).
I have one question, though:
Do you guys recommend flyback resistors to protect the EL34 tubes / OT? Are there any risks putting a 1N4007 from anode to ground on those two?
Well, enough talking. Heres the updated schematics:
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Ok.
After all the measurements in the Oscilloscope thread, heres the updated schematics.
One of the nearest day, I will adjust bias a little bit warmer while I run a signal and check with the scope if those spikes vanishes. If not, maybe I will go further down on those coupling capacitors before the EL34s. Some say 22nF is better? But then I`ll have to order some. Don`t have 22nF with an acceptable voltage rating.
Maybe, for safety of my OT, I will go up a bit on R27 on the PS to lower the screen a little bit. But all these are thoughts I have. Any thoughts here?
And yeah, I know I should make more measurements. Just a little lazy, and since I don`t have a permanent work shop, I must each time connect all the instruments (I am working at our living room table).