Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Patrik on November 14, 2020, 03:04:26 am
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Greetings,
I am a novice here and am currently in a process of building my custom amplifier for the first time. I made a few replicas, Fender 5F2a and Vox AC4, but wasn't fully satisfied with the sound and feel, and am now designing my custom one from scratch. The goal is to have an amplifier that is as simple as possible, single ended and only gain, tone and volume fir controls, light and portable, made so it can still do effects like echo and fuzz, and rich in midrange and lower highrange, being able to saturate it only using the volume knob on a guitar.
Now I settled on EF86, first stage, ECC83, second stage, EL34/6L6/KT66, power tube, and EZ81 rectifier. I had no problems designing the actual amplification circuit but power supply is another matter. I don't understand how filter caps values ate calculated. I don't want to use choke, to save weight, and will be using Hammond 372BX or 373X as a power transformer and Hammond 125DSE as an output transformer.
Any help on that matter will be greatly appreciated.
Thanks in advance,
Patrik
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https://el34world.com/Forum/index.php?topic=15436.0
http://www.valvewizard.co.uk/smoothing.html
http://www.bustedgear.com/faq_Amp_recap.html
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Thank you.
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Thank you.
You are welcome.
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Here's some controversial thoughts:
I have done some thinking about the single ended amps I built and why I was never fully satisfied with the sound and feel. I think I always missed the solid bass frequencies that come with almost any push-pull amp. I tried bigger transformers, 6L6GC power tube, higher supply voltage, bigger speaker and cabinet. Heard great mids and distortion. But empty bass. Then there is the weight problem. Single ended transformers are significantly heavier than push-pull of the same wattage. Many builders here seem happy with single ended designs. I predict they will share their experiences too. :grin: Their dream amp sound is different than mine. Based on your brief post I'm wondering if you might be happier with an amp with push-pull power tubes.
If I've misread your interests and you want to go forward with your plan, consider using a 5xxx rectifier. That would better match the heater options of your intended PT. Happy building!
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I build a single end amp 15 years ago because I have lots of "very good" PP amps. I don't need another one.
And it is a fun project,
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I have played every type of amplifier there is but wasn't fully satisfied with anyone due to my tendency to control gain and volume via tone on volume on guitar, and many modern amps are built so they can take all sorts of pedals without a problem. The closest I got to the amp I like was a Dynacord KV10, I liked the response a lot, but I didn't like the overdriven sound even when it was retrofitted with a Celestion G10 Greenback. For now I just want to experiment a little, just for the fun of it and if having fun isn't enough of a reason I don't know what is.
Also if I will be at least semi-satisfied I will still try with the same preamp, that I moddeled after VOX AC15, combined by a tad of 5F2a.
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http://www.duncanamps.com/psud2/
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...I don't understand how filter caps values ate calculated....
Mostly not calculated; use the same values which worked before. Adjusted if they didn't work so well (or "too well").
Light weight and SE 6L6/EL34 may not go together. (Commercially this was always a very rare combination, even in fixed-install amps.)
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I have a SE amp I built... actually, I have rebuilt it many times, and I just don't like it as much as the push-pull amps I have built.
I used a 125ESE OT. I've used a 12AX7, 6SJ7 (pentode) and 6L6 in the latest incarnation. I've changed things many times and don't remember where the circuit stands, but I don't desire to play it much. I hope you have better luck.
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I've used a 12AX7, 6SJ7 (pentode)
that's enough gain to trash PSE KT88's
try setting the VAC rms to about 1.5X the cathode vdc
is the OT UL? (I don't want to look)
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I don’t want to highjack this thread...
It was an experiment. My original goal was to get the pre-amp pentode to overdrive and then amplify the distorted signal with the class A PA. There were some good things and some bad things that I learned.
Can you explain setting the VAC?
Once again, sorry if this is distracting from the OP’s thread.
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If you want to use a rectifier tube, you MUST follow the rectifier tube specs concerning the filter caps values.
Check out the data sheet for that particular tube
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Thank you guys, You have been much help to me and this is is what I came up with.
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Nice sketch,
Why EL34 ? Is it not too powerful and expensive tube for low power amp ?
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I would like somewhere around 10W of output and tubes like EL34 and 6L6 in SE configuration can supply that. And they are rather cheap here in Europe.
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I would like somewhere around 10W of output and tubes like EL34 and 6L6 in SE configuration can supply that. And they are rather cheap here in Europe.
How can you know you will have 10 watts ?
EL 34 is cheap here too.
EL84 is cheaper but only 6 watts
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They are rated to max out at 10W.
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They are rated to max out at 10W.
If you feed it with the right preamp.
With only one preamp tube you will not have more than few watt
What you say is like, I use 100 watts speaker so my amp power will have 100 watts output :laugh:
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I don't think that number of preamp tubes has such an effect on the output power of the amp, since most amps have only one preamp in SE and one preamp and phase inverter in PP configuration. I am far from an expert when it comes to how the amp works, but I still think that the preamp I designed will be sufficient to power one EL34 up to 10W.
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since most amps have only one preamp in SE and one preamp and phase inverter in PP configuration.
I never see these amp with more than 2-3 watts output.
Can you show me one ?
Update ;
This one ; 10 watts : 1 X 12AX7 and 1 X 6L6 or EL34 :BangHead:
I have always somthing to learn. :laugh:
http://www.allenamps.com/classic10.html
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Thank you guys, You have been much help to me and this is is what I came up with.
Your coupling and decoupling cap values are ginormous, orders of magnitude greater than is desirable for an instrument amplifier, more appropriate for a seismometer :icon_biggrin:
There are many instrument amps with a single stage preamp (in one channel at least), just requires reasonable gain from the power amp and a sensible definition of which stages fall under which category.
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Power supply voltage must be ( very ?) high too.
Not sure 300 volts is enough and enough curent with this rectifier tube
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I can go up to 350 with EZ81 what should be sufficient, but EL34 is rated for 250V as I can see on the specsheet, so I don't see a need for a higher voltage transformer, please correct me if I am wrong. About coupling caps, I am using the same value as is standard on Fender amps, but will be happy to change them if you thing that change would help.
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And I got the idea for for such an amp from Weber Maggie.
https://tedweber.com/maggie-c-kt/
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And I got the idea for for such an amp from Weber Maggie.
https://tedweber.com/maggie-c-kt/
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Yes.
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Thank you guys, You have been much help to me and this is is what I came up with.
Why do you have two different plate voltages on V2?
Typically same tube would use the same plate voltage.
Also why did you design with three gain stages before your power tube?
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For the plate voltage I don't really have an answer. Probably just becouse I am used to seeing resistors between filter caps. And I would like a bit more gain than a single 12AX7 can provide.
I am not an expert in designing amplifiers what is the reason why I begun this post and would appreshiate any suggestion and opinion.
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Thank you guys, You have been much help to me and this is is what I came up with.
Another thing is that it's essential to provide a dc path between a tube's control grid and cathode, ie the grid must be 'referenced' to the cathode. Usually that's done via a gain pot or grid leak resistor, with cathode bias, both would be referenced to circuit common 0V.
Your first and second stage's grids are missing such a reference.
The 100k input grid stopper will cause a lot of hiss; pentode input stages generally don't need a grid stopper.
Regarding the coupling cap values, they work with the resistance in the circuit to provide a high pass filter, whose corner freq is determined by the cap's value C and (total effective) resistance R values. Such that F = 1 / (2 x pi x C x R)
In the old days, a useful 'rule of thumb' (ad hoc guideline) was that for a 'full bandwidth music' (50Hz-15kHz) reproduction, C (in uF) x R (in k ohms) should = about 10 (to my recollection, the 20Hz-20kHz 'hifi' standard was more of a 70s transistor era thing).
That gives the filter's -3dB corner freq of about 16Hz, so minimal attenuation of any guitar bottom end (a bass amp may need a bit more, eg CxR = 20).
These days, using an online calc is easy, eg http://www.sengpielaudio.com/calculator-RCpad.htm but the old CxR=10 is still a pretty handy 'hack'.
So if a stage is feeding a 1M pot (ie 1000k), a 0.01uF coupling cap may be a good place to start. If, having tried that, the bottom end needs 'tightening up', the cap's value might be reduced to 0.001uF; maybe that's too far and 0.0022uF will be settled on, or there's still too much bass, and perhaps 470pF will work out best.
With guitar amps, they tend to be overdriven, and if the above 'CxR' value is too high, a big problem is that if the stage being fed gets pushed into grid conduction (which typically defines the onset of overdrive), the Vac of the signal becomes rectified and the grid's reference Vdc gets pulled negative below 0V. This is referred to as 'bias shift'; some degree of it is inevitable and sounds fine, but too much and bias gets closer and closer to cut off, the tone gets farty, until the whole signal gets momentarily choked out, referred to as 'blocking distortion'.
Old Fender designs often used a CxR of about 20, and perhaps up to 50, hence eg a Tweed Deluxe can get farty and typical mods include reducing the coupling cap values.
And over the 60s, as Marshall gradually tweaked the 5F6a based design they started out with, some key coupling cap values dropped in value, eg the CxR of the coupling caps to power tubes reduced from 20 to 5 (JTM45 to JMP 1987 / 1959).
Aiken's tech info pages are really useful reference source, written by a proper EE but reasonably easy reading / not too daunting, eg http://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers
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Thank you a lot. I didn't even notice problems with coupling. Thanks a lot.
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I hope that fixes problems with catode coupling. I also changed the coupling caps.
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I hope that fixes problems with catode coupling. I also changed the coupling caps.
Still missing a grid return resistor for that triode.
And 1000µF seems excessive for a cathode bypass on that EF86. Where did you get that value?
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I got that cap value from diagram linked below (#1). The second one is what I think might be better based on your comment.
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Thank you guys, You have been much help to me and this is is what I came up with.
Regarding the HT supply -
From p4 of https://tubedata.altanatubes.com.br/sheets/010/e/EZ81.pdf when feeding 300-0-300 into an EZ81, the Vdc at the reservoir cap is more likely to be about 350V.
With the first node / reservoir as 'E' and the last node as 'A', by the normal conventions on such matters, your HT nodes are labelled backwards :icon_biggrin:
The last node (A) is missing its decoupling cap.
The EZ81 can cope with a higher reservoir cap value; why not make use of that capability, as with SE amps, hum from HT ripple is a problem. Hence I suggest 47uF for the reservoir cap, or at least 33uF.
For other values, I wouldn't use less than 22uF unless specifically copying a vintage design.
Note that SE amps greatly benefit from the use of a CLC / pi filter in the HT supply to the OT / plate, eg 5E1 https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5e1.pdf as the reduction in HT ripple, and hence hum contamination of the output signal, can be significant.
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I got that cap value from diagram linked below (#1). The second one is what I think might be better based on your comment.
Those circuits are intended hifi type applications.
This calc for cathode bypass is accurate and complies to the method in RDH4 http://bmamps.com/CapCal.html
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It might be easier to put another 12AX7 in the amp that would replace the EF86. Then I could also move controls, so that gain would be between first and second stage and tone between third and fourth. How does that sound to you? My reason for using an EF86 is purely due to it being a bit more exotic than any old 12AX7. But I am much more familiar with with double-triode tubes than pentodes for preamp. Maybe it would be good idea to use 12AX7 so that I don't try too many new things at once. I think that making a self-designed amplifier is enough of a job in itself.
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So, there is a completly revised diagram of the amp. I replaced EF86 in the first stage with 12AX7, put gain and tone control between first and second stage, but power amp remained unchanged. I also made aa few changes to power supply and heater wiring.
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Still missing a grid return resistor for that triode.
actually missing two now.
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Where? Do you mean grid leak resistor?
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triodes 3 and 4.
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Honestly I have no idea where to put them. Help please.
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'tween the coupling cap and the grid
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Thanks, would 68k do? Also, does each triode has to have its own grid stopper resistor? I thought that one on the input would be enough.
Update: just found out that 40k would do.
http://www.valvewizard.co.uk/gridstopper.html
Thanks for noticing.
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Not talking about grid stoppers! Connect one end of a 1 meg resistor to the grid pin 2. Connect the other end of that resistor to ground. Call it a grid return resistor. Some people call it a grid leak resistor. Repeat for the other grid pin 7.
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Ok, thank you.
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I've built 2 different SE output amps, both using a 6L6 for the output tube. Both amps use a PT and OPT from small PA amps. Both amps use a 5Y3 rectifier, as per the original amp the transformers came from. Both amps are a variation of the Fender Champ circuit. On the first amp I used a bass and treble control instead of a single tone control. On the second amp, I used a treble, mid, bass control setup.
I actually like the sound of the first amp better through the same speaker. Both amps were built as little heads, not combo amps.
I can see where a simple tone control could have an appeal for a simple amp.
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The main reason for gain, tone, volume control setup is due to my playing style, that I control my volume and drive via controls on the guitar, without any external effects, sometimes I use tape delay or fuzz but it is only for tone and not drive. The benefit of a simple contol setup as I see it is also for gigs. Before all that corona I used to play live almost every weekend using either Dynacord KV10 or Echolette Echobell. KV10 has only volume and tone and Echolette has volume only with push-pull treble cut on each channel, has four. The other guitarist in a band had some Fender, I think it was Frontman 212, and used at least 30min setting up his pedalboard and making tweaks to his sound, I was done in 2min. So for me simpler is better.
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That does make sense to me.
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I would like somewhere around 10W of output and tubes like EL34 and 6L6 in SE configuration can supply that. And they are rather cheap here in Europe.
How can you know you will have 10 watts ?
EL 34 is cheap here too.
EL84 is cheaper but only 6 watts
see attached. pg. D1, closer to 11W with the published spec.
--pete
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Ok, this might just be it. In the third revision the amplification circuit hasn't changed much, I added those grid return resistors that were bugging me for all that time. Most notable change is in the power supply circuit that lost its EZ81 tube, being replaced by more powerful GZ34 so I can utilize 5V transformer winding and reduce load on heater winding.
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The HT voltage is way too high, 350-0-350 Vac with a GZ34 will be more like 450Vdc.
Where are you getting your numbers from? The load line is horrendous, plate dissipation way over the limit!
A 100pF bypass cap won't have much effect.
The preamp gain is very high, some provision for attenuating the signal level between stages should be considered.
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The LED will not work as drawn. Replace the bottom diode with a 1K resistor for current limiting (use smaller resistor for brighter LED). Be sure the cathode of the top diode is connected to the anode of the LED.
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You were considering the Hammond 372BX 300-0-300 in your first post. How would that work?
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So, at first I was discussing two possible power transformers, Hammond 372BX and 373X. At first I wanted to use the 372BX due to its lower output (cca. 600V) but when I was notified that EZ81 can handle up to 350V and that this might improve the amplifier I switched to 373X (cca. 700V). Now I still had 372BX in my mind and that is where the false input in to rectifier tube comes from. Also I didn't expect that with GZ34 I might get such a boost in output voltage, therefore I will probably change the transformer once more, so I get between 350V and 360V on the reservoir cap becouse as I can see GZ34 at 300V input will put out somewhere about 330V and if possible I would like to stick to current voltages so I don't have to completely redesign power supply, that might be a good idea becouse as you said the load line is rather poor and would benefit from redesign. Also how do I fix plate dissipation? I didn't check the characteristics for GZ34 (it was a mistake) and just assumed it will respond in the same way EZ81. Before that I also considered using 5Y3GT but abandoned the idea due to its lower capacitance rating for reservoir cap, 37uf. Thanks also for noticing the problem with LED driver circuit and that bypass cap is poorly written, it is actually 100uf.
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... the load line is rather poor and would benefit from redesign. Also how do I fix plate dissipation?...
Draw it onto the plate chart, see http://www.valvewizard.co.uk/se.html
This is a nice online tool, once you understand how to use it http://bmamps.com/ivds.html
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But is the amplification part of the amplifier ok? Just so I know if I have to fix something or if I can use it as a starting point.
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But is the amplification part of the amplifier ok? Just so I know if I have to fix something or if I can use it as a starting point.
pdf64 said, "The preamp gain is very high, some provision for attenuating the signal level between stages should be considered."
4 gain stages without any attenuation (voltage dividers) between stages seems excessive to me. The amp will probably be a squealer. Be prepared to put a voltage divider between every stage. Here's an example of a 4 stage preamp with voltage dividers to tame the gain...
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Thanks for advice.
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And what if I used some lower gain tubes on third and fourth stage? Something like 12AY7 or maybe even 12AU7.
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How 'bout if you only use one tube?
What's your goal in using four triodes? Gonna play some big hair music?
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I want to have enough gain to push power tube into fuzz, over the overdrive. I am not sure if one 12AX7 will do.
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I want to have enough gain to push power tube into fuzz, over the overdrive. I am not sure if one 12AX7 will do.
With a simple vol / tone arrangement, I think it probably would, see
Likely schematic https://el34world.com/charts/Schematics/files/Fender/Fender_princeton.pdf
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I want to have enough gain to push power tube into fuzz, over the overdrive. I am not sure if one 12AX7 will do.
You're on the right track. 4 triodes will probably put out square waves for any guitar you plug in. Definitely gonna do fuzz. Controlling all that gain is gonna be a challenge.
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you have an SE amp, which by design wants to be Class A, A1.
They can fuzz out, but they can also scream, make treble totally annoying and leave Bass farting all over the stage.
This just a guess-tim-it, the cathode VDC will land somewhere 15-25vdc (your bias). Once you know that number "design" the pre to give the PA tube somewhere between 30-50 volts AC signal with gain n volume at 10.
then play it, then modify it closer, then repeat.
or
take a known good amp that you like the fuzz and "build around that design" adding your twist
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For now I found out that I enjoy the sound of any amp without negative feedback much more than with negative feedback. Maybe doing removing NG and using that Maggie amp circuit that was mentioned previously on this thread would be a good starting point, eliminating the need to do from the scratch designing and give some foundations that hopefully work, to build up from there becouse as I see it I can always take the amp apart and modify it. What I want to say is, that I will try to find some suitable 10W amplifier circuit, SE or PP, doesn't really matter, and start from there and we will se how it goes. Now I have in mind previously mentioned Weber Maggie and Fender Princeton Blackface (with that I will have to find a way to remove vibrato and reverb circuit). Thanks a lot to all of you who responded.
Update: I went to have a little look and seems I am not the only one with the idea of reverbless and vibratoless princeton. Rob Robinette made such a modification and can be found in the link bellow. He used a dummy load to immitate the effect of reverb on the rest of the circuit. I think I read somewhere that reducing that load would increase the drive emmensly so replacing it with some lower resistance resistor might be viable option to get more drive, or maybe even putting in a potentiometer. Modifying the tone stack to volume tone arrangement and loosing negative feedback and this could just be a winner for me. I have always loved the sound of overdriven 6V6 tubes so this is a very intriguing option. I also have stacks of them lying around. Loosing the bright cap would also be nice I think becouse I intend to use Jensen P12R speaker which is rather bright so balancing it with a bit darker amp would de a good idea I suppose.
The amp explanation page: https://robrobinette.com/AA1164_Princeton_Reverb_With_Reverb_and_Tremolo_Deleted.htm
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Hello to all once more. After I did some research I found out that, as someone indicated before, weight isn't the SE amps strong point, kinda counter-intuitive but anyway, after I posted that diagram of a modified Fender Princeton yesterday I made a little research and found out that the weight of the output transformer for that amp is one third that of a SE 10W amp. Therefore I made a decision to switch po PP amplifier, so I wave quite some weight and also cost, transformers are cheaper by quite some margin. Now I settled on diagram by Rob Robinette for Fender Princeton but without reverb and vibrato. I already made some modifications and I think that for now at least that is what I will go with. The changes I made are, modified tone stack, so I have only Tone control, tone and volume control bypass, reverb load was replaced by a potentiometer (it turned out I was correct on that that smaller load will cause more gain, therefore I decided to replace resistor with a 500k pot), I also lost bright cap that was originally there and put on a pre-phaseinverter master volume. For transformers I settled on Hammond 376X power transformer and 1750E output transformer, which is a direct replacement for original Princeton one. I was just wondering if I could replace GZ34 rectifier with 5Y3GT, since it still should be powerful enough for this amp.
For now this is all from me and I would like to hear some response from someone with some more experience than I have. Thanks to you all.
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Your LED and diode will both fail immediately when you apply power.
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I think the new schematic is getting close. But the PT is still way too big. The 376X is rated to provide 640v CT @173ma. The amperage rating for your design should be around 100ma or the resulting DC voltage on the power tube plates will be way too high. I reread your first post where you noted that your Dynacord KV10 was the close you what you want. Very interesting amp--I think I would like it a lot. It is rated at 10 watts and has a B+ of only 220 dc volts. Amps with B+ over 400 volts don't get power tube distortion without being really loud. I think a push pull amp with B+ about 350 dc volts might be closer to what you want. The Hammond 290AEX is rated 550v CT @ 100ma using the low voltage secondarily taps. I use the USA version with a 5Y3 and a 1750E to get B+ of 350 in my Princeton Reverb clone. When I use the high voltage secondaries I get 390v with a 5Y3. Varying rectifier tubes and secondary taps allows great leeway in finding the right B+. I'm assuming that going from 220v B+ in the Dynacord to 350v in a Princeton will give a more crunchy distortion sound without losing your ability to adjust it with the guitar volume control. I think it would be close enough to allow parts-tweaking to get your ideal performance--until you "need" another amp. :icon_biggrin:
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In this case I can probably replace 376X transformer with something of lower voltage and power rating.
And for now I will let go of LED and just use usual wolfram bulb. Probably my best option then is a Hammond 372BX transformer, which is a bit lower voltage, but if it was wired as for 220Vacc or maybe even 200Vac with modern 230Vac it is probably the closest I can get to original specs, without actually using Princceton replacement transformer (I do't like Fender replacement transformers since they need a huge hole in the chassis to be installed, much prefer the upright type).
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Hallo, to you all. Just a little update on my project. Atter a bit of thinking i decided that ordering a precut chassis would be much easier than modifying and using a general purpose chassis. So I designed a small chassis, so all components can be wired point to point, plans are attached bellow. Don't bother reading becouse it is written in slovenian, so just if someone else might be interested. I am having chassis laser cut and costwise this is nearly cheaper than using universal chassis.