• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12AX7 EL34B TUBE AMPLIFIER

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Well after months doing searches and reading I have come up with this amp and it sounds really good too. I did the schematics hand drawn as I went along recorded all voltages and I hope it is right. all voltages as I can see are in the range of all tubes. I'm running a media player through the amp and playing FLAC recordings and wow forget MP3 the quality of FLAC out strips any MP3.
I hold no responsibility for the schematic up to you if you decide to build it seems to work very well for me and no hum as I use DC for all heaters in the tubes. let me know what you think if you see anyyything I have done wrong so I can fix. pins 8 and 1 of the el34b are joined together ONLY.
 

Attachments

  • 100_6833.jpg
    100_6833.jpg
    752.4 KB · Views: 744
  • 100_6834.jpg
    100_6834.jpg
    680.6 KB · Views: 630
  • 100_6835.jpg
    100_6835.jpg
    830.3 KB · Views: 609
  • 12ax7 el34b valve amp schematic.jpg
    12ax7 el34b valve amp schematic.jpg
    421.2 KB · Views: 731
Last edited:
Your design is not optimised. You may like to consider the following points:-

1. The cathode bypas capacitors are generously sized. The LF cutoffs are V1 (12AX7) K ~ 1Hz, V2 (EL34) ~7.2 Hz. The 12AX7 cathode bypass in particular can be much smaller without any audible change. But then there is no real disadvantage either.

2. The grid coupling cap feeding the EL34 is way too big, with an LF cutoff of only 0.7 Hz. You cannot hear anywhere near that low. The overload recovery characteristics of an amplifier are important, especially with amp having low power output such as this. Even at modertae volumes, any amp is driven to clipping occaisonally on music peaks. And on scratches if palying from vinyl. When the tube is overdriven, it draws grid current. The gid current charges the coupling capacitor, so the output tube is overbiased and parallised fro a short time after the overdrive peak has gone. Most people find it difficult to detect the occaisonal clipping lasting milliseconds, but the paralysis following it can be audibly obvious.

To address this, increase the 12AX7 anode resitor and decrease the grid coupling capacitor. Increasing the 12AX7 anode resistor won't change teh gain much as the anode resistance of a 12AX7 is low. I recommend 220 Kohm and 10 nF.

Large grid coupling caps are only necessary when global negative feedback is used.

3. Is the transformer specified for SE operation?

4. You have shown ultralinear operation. Ultralinear works best in push-pull, as it tends to increase even order distortion which is cancelled out in push pull. You willl get cleaner, lower distortion sound, by not using ultralinear mode and instead bring the gain back to teh same level with local negative feedback anode to grid (a high value ressitor between the anodes of the 12AX7 and EL34. Normally this increases hum, but you have plenty of filtering.

5. You may be interested in a trick used by many commercial SE designs in teh 1940's and 50's. They fed the output valve screen from the same HT supply as the driver anode. The supply ripple fed to the driver anode and output screen is cancelled by the ripple fed to the output anode if conditions are set up right. This allowed them to get away with less power supply filtering. The lower ripple in the screen and driver supply was adjusted by filter design so that the output tube gain brings it up to equal and opposite the ripple of the anode supply.

6. My back of the envelope calculations indicate you have tube operating conditions pretty right. It should sound nice, apart from the overload recovery.
 
Last edited:
Administrator
Joined 2004
Paid Member
The plate load resistor for your 12AX7A needs to be significantly larger than the rp of the triode it is loading to minimize distortion. In this case I would not go less than 100K, however the driver source impedance will increase substantially which depending on strays and input capacitance may cause a premature HF roll off, miller capacitance in UL is not an issue, but experiment with triode connection and miller will be a big issue with this driver correctly set up. It seems that this circuit might actually work a lot better with a 12AT7A/ECC81/6201 which will be much happier driving that 22K plate resistor.

There is no benefit to DC heating the EL34 filaments, the filament voltage is approaching the upper limit of acceptable and the transformer is working harder than necessary. The filament voltage on the 12AX7A strikes me as a bit low - even here DC heating is not required with careful attention to layout.
 
The plate load resistor for your 12AX7A needs to be significantly larger than the rp of the triode it is loading to minimize distortion.

I was a bit startled when I read your post. Then I realised that I read the wrong line in my tube databook and the rp of the 12AX7 is much greater than I thought it was.

Thus increasing the anode load resistor, as we both recommended, will increase the gain, and this may be a nuisance to the OP. If this is the case, he should keep the 12AX7 load resistor fairly low, and increase the EL34 grid stopper to reduce the amount of grid blocking on music peaks.
 
Mdamp,

Your new amplifier looks very nice in the photos with regards to craftsmanship and layout. There is a couple of things you might want to think about. One, I noticed that you did not use any rubber grommets in the chassis where the output transformer wires pass through to the bottom of the chassis. Rubber grommets are a good idea in this location as the B+ and plate lead going to the EL34 are elevated to ~320vdc. The second thing I do not see in the photos or the schematic, is a line fuse. Maybe you have incorporated one on the underside of the chassis and disregard this if you did. Otherwise the amplifier is not protected from serious over current situations. Keit and Kevin have offered some good advice to consider too. With regards to DC filaments, my experience is they are not necessary on a power amplifier. I have built many pp and single ended tube amplifiers and have never had any significant issue with hum. The last one just completed, I would describe as virtually dead quiet with not signal applied. The only place I have ever needed and used DC filaments is in very high gain situations such as phono section pre-amps. Great job on you amp and good luck with further improvements! Mickeystan
 
Last edited:
Well after months doing searches and reading I have come up with this amp and it sounds really good too. I did the schematics hand drawn as I went along recorded all voltages and I hope it is right. *****************************.

The project is beautiful and congratulations, you can try these small changes and you will surely have better performance, you should work with optimal PSF tube 12AX7 (ECC83).
Changes: R5, R6 and R12 feedback of 2.2Meg, but not less than 1Meg, value practically attempted.
From R12 amplifier gain is established.
 

Attachments

  • SE EL34 DIY-AUDIO.png
    SE EL34 DIY-AUDIO.png
    47.4 KB · Views: 641
Last edited:
The project is beautiful and congratulations, you can try these small changes and you will surely have better performance, you should work with optimal PSF tube 12AX7 (ECC83).
Changes: R5, R6 and R12 feedback of 2.2Meg, but not less than 1Meg, value practically attempted.
From R12 amplifier gain is established.

This implements global feedback. If you are going to have global feedback, take the feedback from the output transformer secondary, not the primary. It makes not a lot of difference LF stability-wise, is effective against transformer distortion, and does not require a high value high voltage resistor (which costs more and is a reliability issue). However it may be a greater risk wrt HF instability - depends on transformer quality.

However, I assumed the OP wanted to avoid global feedback, which is a wise approach for a beginner to take. For beginner, local feedback (anode to anode) may be better, even though it requires a high value high voltage feedback resistor. The resistor is not stressed as much as with feedback to the 12AX7 cathode though.
 
Schade feedback amplifier anode-anode tube can be good preamp tube anode and cathode end.
Recommended configuration cathod tube preamp is final anode will have a harmonic spectrum that H2 will be favored and will linearize final amplifier tube.
Resistance R12 should not be a problem in practice can connect two or more series resistance.
Do what you feel is best, I just suggested something that is sure to give good results.
OK, I understand you do not want a negative feedback on the project.
I apologize if you already know the details!
I wish you pleasant afternoon!
 

Attachments

  • R12.png
    R12.png
    6.3 KB · Views: 290
Last edited:
Keit said:
Increasing the 12AX7 anode resistor won't change teh gain much as the anode resistance of a 12AX7 is low.
On the contrary, increasing the anode resistor will change the gain significantly because the anode impedance of a 12AX7 is high. It may be that the current arrangement is achieving some partial cancellation of second-order distortion in the two stages, but this will be at the cost of more higher order distortion. Two options:
1. increase gain and reduce distortion for the 12AX7 by increasing the anode resistor, then perhaps add some negative feedback - as others have suggested;
2. use a lower mu valve for the first stage, so the low value anode resistor does less harm.
 
On the contrary, increasing the anode resistor will change the gain significantly because the anode impedance of a 12AX7 is high.
True, but did you read my second post?

It may be that the current arrangement is achieving some partial cancellation of second-order distortion in the two stages...
In cascaded transistor common emitter stages, this is a well known dodge - you scale the emitter current in the two stages right (theoretically in proportion to the signal level in each stage, in practice the ratio needs to be somewhat diffrent due to transistor manufacturing process changes), and the 2nd harmonic is completely cancelled.

However in tube circuits, while some cancellation occurs, it cannot be optimised like in a transistor amp, and the effect is too small to be worry about. Noted English engineer T Roddam in the 1940's proposed using positive feedback around the driver to get better cancellation and make global feedback more effective, but the idea never caught on for obvious reasons.

The 2nd harmonic cannot be cancelled in a cascaded tube amp because tubes are voltage driven, and the transfer characteristic is determined almost entirely by tube geometry and only to a very small degree affected by operating conditions. Complete cancellation is possible in transistor amps because the emitter current/base input transfer is a logarithmic relationship over a wide current range - so you can keep the percent signal amplitude a constant ratio to emitter current and get the same level of harmonics.
 
Keit said:
True, but did you read my second post?
Sorry, I have now; my apologies.

However in tube circuits, while some cancellation occurs, it cannot be optimised like in a transistor amp, and the effect is too small to be worry about.
Yes. I wasn't advocating the practice - although others on here sometimes do - but just noting that it might be happening accidentally, so reducing stage one distortion might increase total 2nd by a little. Not a problem, as 2nd is relatively benign (or even preferred by some SE fans) and higher orders matter more.
 
Theoretical simulation, the differences are the pictures, model OT and vacuum tube are the same in both cases.
 

Attachments

  • FFT SE EL34 schema originala.png
    FFT SE EL34 schema originala.png
    44.3 KB · Views: 265
  • FFT SE EL34 cu feedback.png
    FFT SE EL34 cu feedback.png
    44.6 KB · Views: 261
  • SE EL34 BODE  schema originala.png
    SE EL34 BODE schema originala.png
    38.7 KB · Views: 264
  • SE EL34 BODE cu feedback.png
    SE EL34 BODE cu feedback.png
    40.2 KB · Views: 255
Theoretical simulation, the differences are the pictures, model OT and vacuum tube are the same in both cases.

This is meaningless.

It is meaningless because you haven't described how the simulations were done - with what tool and with what tube models. And what transformer parameters. Exactly what simulation circuit was used?

Common sense will show that distortion will be different depending on the degree and type of feddback. If that is not what your simulation showed, then quite simply your simulation is wrong.

In this case, as the OP grossly oversized the capacitors, and the 12AX7 anode load is very low, the frequency response is determined solely by the output transformer in any reasonable choice of transformer - you don't need any fancy simulation to know that.

If you used a SPICE tool, be aware that SPICE is unreliable in predicting distortion for ultralinear circuits, as what happens in the output tube is complicated, being determined, amongst other things, by the grid/anode transfer characteristic, the grid/screen transfer characteristic, and teh grid/anode transfer characteristic, all of which are non-linear in differing degrees.
 
This is meaningless.

It is meaningless because you haven't described how the simulations were done - with what tool ********************************************************************.

The simulation was performed with the same models and OT tubes in both cases, so I wanted to show that there is a difference between the two projects.
If you justice is perfect, I spousal just my opinion.
OK!
and these are the models:
*******************************
.SUBCKT EL34B A S G K
*
* Calculate contribution to cathode current
*
Eat at 0 VALUE={0.636*ATAN(V(A,K)/23)}
Egs gs 0 VALUE={LIMIT{V(S,K)/9.3+V(G,K)*0.95,0,1E6}}
Egs2 gs2 0 VALUE={PWRS(V(gs),1.5)}
Ecath cc 0 VALUE={V(gs2)*V(at)}
*
* Calculate anode current
*
Ga A K VALUE={1.86E-3*V(cc)}
*
* Calculate screen current
*
Escrn sc 0 VALUE={V(gs2)*(1.1-V(at))}
Gs S K VALUE={1.518E-3*V(sc)*LIMIT{V(S,K),0,10}/10}
*
* Grid current (approximation - does not model low va/vs)
*
Gg G K VALUE={PWR(LIMIT{V(G,K)+1,0,1E6},1.5)*50E-6}
*
* Capacitances
*
Cg1 G K 15.4p
Cak A K 8.4p
Cg1a G A 1.1p

.ENDS EL34B
***************************
.subckt RCA12ax7 1 2 3
*************************
eGIogVpc 20 0 value={log(v(1,3))}
rGlogVpc 20 0 1
eG0 10 0 poly(1) <2,3> -3.7694e+00 1.9947e+00 5.9432e-02
eG1 11 0 poly(1) <2,3> -3.2024e-02 -4.1443e-02 -4.8236e-03
eG2 12 0 poly(1) <2,3> 1.9127e-02 -1.2189e-02 -1.5526e-03
eG3 13 0 poly(1) <2,3> -1.1354e-02 4.9339e-03 6.1016e-04
rG0 10 0 1
rG1 11 0 1
rG2 12 0 1
rG3 13 0 1
gG 2 3 value={(exp(v(10)+v(20)*(v(11)+v(20)*(v(12)+v(20)*v(13)))))/170}
*
eP0 110 0 poly(1) <2,3> -9.9158e+0 1.9145e+0 -2.8135e+0 1.8661e+0
+ 1.5643e+0 4.7240e-1 6.4276e-2 3.3101e-3
eP1 111 0 poly(1) <2,3> 9.5428e-1 3.2558e-2 -8.3349e-1 -4.8578e-2
+ 2.6213e-1 1.0492e-1 1.8921e-2 1.3632e-3
eP2 112 0 poly(1) <2,3> 9.5766e-2 2.5192e-2 2.2391e-1 -1.7040e-1
+ -2.4952e-1 -1.0960e-1 -2.0981e-2 -1.4882e-3
eP3 113 0 poly(1) <2,3> -6.6107e-2 -3.9657e-2 7.5560e-2 3.1025e-2
+ 2.4265e-2 1.7002e-2 4.2512e-3 3.4761e-4
eP4 114 0 poly(1) <2,3> 8.4148e-3 4.7989e-3 -1.3258e-2 -1.9288e-3
+ 5.2888e-4 -5.6853e-4 -2.4727e-4 -2.4359e-5
rP0 110 0 1
rP1 111 0 1
rP2 112 0 1
rP3 113 0 1
rP4 114 0 1
gP 1 3 value={(exp(v(110)+v(20)*(v(111)+v(20)*(v(112)+v(20)*(v(113)+v(20)*v(114))))))}
Cgc 2 3 1.8p
Cgp 2 1 1.7p
Cpc 1 3 1.9p
.ends
*****************************
* Sample pspice TRANSFORMER library
* BY Luis Cárdaba
* August 12, 2001
* SINGLE-ENDED_UL 3K5 / 8 OHMS
.SECONDARY a=20.62; a^2=425
.SUBCKT SE_UL_3k5 1 2 3 4 5
LP1 1 2 26H
LP2 2 3 24H
LS 5 4 0.2353H ; SPEAKER 8 ohm SECONDARY
CP1 1 2 .342NF ;CAPS FROM SPECS
KALL LP1 LP2 LS 0.9999975 ; = 1-1/403600
.ENDS
******************************
 
Last edited:
Ex-Moderator
Joined 2011
The simulation was performed with the same models and OT tubes in both cases, so I wanted to show that there is a difference between the two projects.
There is a difference of the distortion with and without the feedback, no surprise there, but the SPICE models that you use for the OPT seems odd, it's nearly a 50% UL tap with a very low LF corner, not that it would impact the THD results at all, curious selection that's all...
 
What 12AX7s are those? can't tell....

And oh God...those Chinese/Shuguang EL34Bs......seen/had some of them literally explode/blow up/meltdown haha.... still have old pair of the older As though their still strong too...sound wise just ugh kinda haha.

Telling you drop some JJ E34Ls in there at least, it will be so much better. Or maybe It's just me but every time used the Chinese/shuguang EL34s in anything...just more Dull/One-dimensional sounding....
vs pretty much every other EL34s characteristic mid grind/aggressive/harmonic swirl etc
 
Last edited:
There is a difference of the distortion with and without the feedback, no surprise there, but the SPICE models that you use for the OPT seems odd, it's nearly a 50% UL tap with a very low LF corner, not that it would impact the THD results at all, curious selection that's all...


OK! thanks for the information, we can use a transformer in Spice and generic, I wanted to show the difference between a normal and one with NFB project .
It's hard to communicate with google translate, so I withdraw from discussions, I apologize.:(
 

Attachments

  • OT SPICE (generic).png
    OT SPICE (generic).png
    12.7 KB · Views: 214
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.