G9 and its frequency response ?? (impedance matching?)

[steppenwolf]

Yes, exactly!
I just finished building that output stage with a 6922 (should be the same as a ECC88). I made most of the connection directly on a seperate noval tube socket outside the box. I fitted everything from the 470k input but didn't load the output.
As B+ I used the +245V already present in the G9. Current draw for one 6922 was around 5mA, a bit more then with the ECC82 but still OK, I think.
Strangely, I didn't get the kathode to sit at exactly 1/2 B+ or around 122V. Mine is around 90 Volts unloaded. When adding a 8k loading resistor it drops to around 77V. I don't know why I don't get 122V ??? ???
The other kathode reads some 2,5V...
Could anybody help me please!

Thanks,
Stefan

 

[Jonte Knif]

I really need more base in my G09 to be able to use it on more sources. With my G7 it is especially bad (because the G7 should have a source impedance round 600 Ohms) so it makes singers sound thin.

You might also, in any case consider getting a 7:1 or 10:1 Lundahl for your G7 some day. (pin compatible) But that is another impedance/transformer issue then.

As B+ I used the +245V already present in the G9. Current draw for one 6922 was around 5mA, a bit more then with the ECC82 but still OK, I think.
Strangely, I didn't get the kathode to sit at exactly 1/2 B+ or around 122V. Mine is around 90 Volts unloaded.

There probably is a typo in the original schematic. You simply do not get the cathode to sit at half B+ if you have different size cathode resistors. They don't have to be equal, it might not be the ideal, but if you don't have time/equipment/6th sense it is a valid starting point.

In my opinion a higher current draw could actually be beneficial. 5mA current with 2:1 output trafo is still kind of lazy. This gives you the absolute maximum output current of only 10 mA. So, scale down those resistors if you dare!

 

[1954U1]

Hello... I just bought input and output OEPs 2 weeks ago for my G9. I'm still building it, and now I'm a little bit confused about the way I need to go... so, what do I need to do to get more bass response and get rid of the ringing high end? swap the input trannys? change the output tubes for the 12AU7s? well... I have no experience just to see the schematic and take some conclusions that far, and I'm still building it so can't test it... so my hand are tight for now and I will follow your indications and experiences
Thanks for all you're knowledge shared here! love DIY!
Eddie ;D

I'm working for now on the subsitution of the OEPs input, today/tomorrow I'll post also the curves with Cahrnill and Edcor inputs at various impedances [40 150 600 Ohms], and some other measurements of  the input OEP outside the circuit and with a scope too for square wave's ringing.
Others here [see posts above] are working also on output stage.

There is also the subjectivity of the sound of the transformers of course.

 

[steppenwolf]

There probably is a typo in the original schematic. You simply do not get the cathode to sit at half B+ if you have different size cathode resistors. They don't have to be equal, it might not be the ideal, but if you don't have time/equipment/6th sense it is a valid starting point.

In my opinion a higher current draw could actually be beneficial. 5mA current with 2:1 output trafo is still kind of lazy. This gives you the absolute maximum output current of only 10 mA. So, scale down those resistors if you dare!

Do you really think it is a typo? The resistors are 1k and 470R on both the schematic and the pcb layout...
Jakob even added that the kathode sits at 125V

Sorry, tube electronics is kind of new for me. Would you think a 470R 470R combo would be ok?

Best,
Stefan

 

[steppenwolf]

Ok, just swapped the 1k against a 470R, now I'm getting pretty close to B+/2!
Current draw now is some 8mA.
Would you think this could perform better?

Best,
Stefan

 

[riggler]

So has changing to the OEP output trannies helped anybody? Looks like there are quite a few fixes. I wonder if Jensen had a good choice here... Will check it out.

 

[steppenwolf]

Well, it all seems to boil down to the following:
Using the Lundahl output with the standard ECC82 topology results in a big bass roll off due to the highish output impedance of the SRPP stage. The Lundahl doesn't seem to be build for that purpose. (Just look at the datasheet, it pretty much says to use as low source impedance as possible for optimum performance)
Then, on the other side, using the OEPs, the output seems to have enough inductance to get the bass through. (Or at least much better then the Lundahl)
Then again using also the OEP input the roll off happens at the input transformer. This time the OEP input is the problem as you can get the response pretty flat but only with really low source impedances.
So when using the standard topology, I think it would be best to use a Lundahl input and the OEP output.

Right now I'm trying an ECC88 output stage to get a beefier output stage and get the couple of bass dbs still lacking with the OEP output.

Back to the ECC88 output:

I'm currently running it like in the G Pultec design but instead of the 1k andode resistor, I'm using a 470R resistor to get the bottom kathode idling at around half B+ and at around 8ma.
Is this OK? @ Jonte Knif ;D
I don't understand why I don't get the voltages seen in the G Pultec design. I mean the ECC82 stage also uses a 1,5k resistor and can't achive balance following your logic but it still works?

Thanks for further advice!
Best,
Stefan

 

[riggler]

Check these out:

http://www.jensen-transformers.com/ln_out.html

3 models here that might work?

JT-123-BLCF
JT-11SS-DLCF
JT-112-LPC

I've read the DW Fearn uses these. Dig that sound.

 

[1954U1]

Tried the OEP input trans with a scope and a generator with fixed output impedance at 50 Ohm.
Output level of the generator [measured at transformer's secondary] set at 0dB at 1kHz.
The optimal load, as I can see, is 17K [square wave well shaped, no ringing, 20Hz-20Khz sine wave flat].
The load specs on OEPs datasheet [optimum load impedance= 6.25k/25k] are more or less right.
Going down with load, there is a noticeable high roll-off. [- 3dB at 20Khz with 12k load, -6dB at 20Khz with 5k load]
At 25k, there is some ringing, some high boost.
Without load, the thing self-oscillate at 27Khz (+16dB).

I still didnt try with 150 Ohm and 600 Ohm input impedances.
These are, btw, the impedances that gives the bass roll-off on the G9, 600 Ohm worse.

 

[Jonte Knif]

I'm currently running it like in the G Pultec design but instead of the 1k andode resistor, I'm using a 470R resistor to get the bottom kathode idling at around half B+ and at around 8ma.
Is this OK? @ Jonte Knif Grin
I don't understand why I don't get the voltages seen in the G Pultec design. I mean the ECC82 stage also uses a 1,5k resistor and can't achive balance following your logic but it still works?

I think you are fine with those values, but surely, you have to live with the fact that they, from a very strict engineering point of view ( mine), might not be the best possible. How ever, the design is any way somewhat suboptimal so there is no need to get paranoid about the bias. You can still lower the impedance lowering the cathode resistor values, but of course your tube will have shorter life. Any how, I would go directly to 10mA, but that's just me.

I didn't quite get the thing about your ECC82 bias. You say that with equall resistors it didn't bias half way? The tube halves can of course be very different. I dunno.

(People might be wondering here how they/someone else didn't hear this or that problem or thinking that if they didn't hear it the design is perfect etc. Certain high end EQ loses 3 dB top end when terminated with 600 Ohms for example. It might not be one or two mastering studios where this is connected to a vintage compressor. "Uh, I need more AIR" "Hey Larry, I think we have to get new tubes for 670, it has lost some top end" )

 

[steppenwolf]

I didn't quite get the thing about your ECC82 bias. You say that with equall resistors it didn't bias half way? The tube halves can of course be very different. I dunno.

I read somewhere that it is recommended to have the kathode at half the supply voltage in a SRPP stage. What I meant was that in the original G9 schematic
there is a 470R kathode resistor and a 1,5k at the anode. I didn't measure the voltage there but following your logic it shouldn't be possible to achive balance there but why would have Jakob chosen this resitor, then?
Today I will test the tube in circuit.

@1954U1
Thank you very much for this information. It would be really interesting to see how it behaves with 150 and 600 Ohm load!
Best,
Stefan

 

[steppenwolf]

OK, just ran the output with an ECC88 SRPP stage.
The DI input was used, (as the normal line/mic input causes bass loss due to the OEP transformer):

Here is the standard output with ECC82:



The waveform is tilted a little bit, but not really seriously. I could life with that, I think.

Now the response with the ECC88:



The bass bumb maybe due to the capacitor, I will try a bigger one. The dents are because the tube is outside the box and connected with pretty long unshielded wires so it is picking up some noise.
What do you guys think?
Looks pretty good to me. The ECC88 is not really running hot (8ma). Moreover, I measured THD at 1kHz with the ECC82 had some 0,5% THD, with the ECC88 some 0,25%. On lower freq (100Hz) the 82 had some 0,9% THD and the 88 some 0,5%.

Best,
Stefan

 

[Jonte Knif]

I read somewhere that it is recommended to have the kathode at half the supply voltage in a SRPP stage.

There is no magic in that, I think. It depends on what you are trying to do. Bigger upper resistor will often lower the distortion at the levels we are dealing here with. Depends on load etc..

But this is not my expertise really. I never had to go into these subtleties because I don't use SRPP in my own gear

Congratulations on your test drive! The bass resonance can (and should) be moved down by increasing the cap value, but it will not go away. You can trim it critically flat with some small series resistor. Try 100-200 Ohms first.

-Jonte

 

[1954U1]

I've done the scope/frequency out-of-circuit tests of the input OEP also with impedances of 150 and 600 Ohms.
Apparently, the freq response at 600 Ohms is much better than at 40 Ohm, its perfect.
And wonderful square wave too.

BUT

the output of the Audio Precision I'm using is always transformer coupled..
so I'm guessing that if the problem of this transformer is lack of inductance,
I should analyze it with a generator not transformer coupled.

The other generator I've used, has 50 Ohm fixed output impedance, and gave the results above.
In some days I'll have here an R&S APN62, with selectable output impedances and selectable outputs configuration [also w/without transf],
so hopefully there will be some answers..

Meantime, I've prepared the G9 with wires so I'll be able to quickly try, in-circuit, other input transformers I have here.

 

[steppenwolf]

Interesting!
Thanks for your work!
Keep us informed;-)
Best,
Saluti,
Stefan

 

[riggler]

Just curious if anyone has tried switching from lundahl to oep output transformers? Does that do enough to resul in a warmer, bassier sound? I could use some color and have read oep might be the better choice for that. Thanks!

 

[1954U1]

Done the freq-level measurements of G9 with Carnhill VTB9045 as input trafo, instead of the OEP.

The bass response is waay better than with OEP, and its the same at 40 150 and 600 Ohms input imped.

So, with OEP as input trafo, there is definitely a mic/line big bass roll-off with input impedances > 50 Ohm.

I'll try tomorrow also the Edcor, and then post all the sweeps.

 

[steppenwolf]

VERY interesting!

So, I think it's time (at least for me) to draw a conclusion:

There are two possible secenarios that will cause  low end loss with the G9 design.

1) When using OEP trannies (as I did) the input transformer that seems to lack inductance causes the problem

2) When using only Lundahl transformers, the input is fine and flat but the output transformer again lacks inductance and causes a pretty huge roll off.
    (This transformer wasn't designed with such a circuit connection in mind, at least according to the datasheet)

So worst case scenario would be to use a OEP input and Lundahl output

Best would be to use Lundahl input and OEP output at least with the normal circuit.

Even with an OEP output, you'll still loose a couple of db. It seems that the ECC82 is really struggling to drive that transformer. That causes the slightly tilted freq response you can see in an earlier post.
Thats why I think I'm going to use the ECC88 in the ouput stage, as it performs so well in my test.
It will run a little hotter then the ECC82, so maybe I'll have to scale down a resistor or two in the B+ power supply.
Moreover, I will change the 4,7uF cap to a 10uF (as I have those in my junk box) to move the bass resonance down.
I also will have to mod the PCB and power the ECC88 with the 6,3V filament it needs.
I will also change the input transformers to Lundahls.
This should suit my needs very well, I think.

I bet just using a Lundahl input and OEP output would do trick but I would like to use the G9 for basically everything, from tracking to mastering and that calls for a flat amplifier.

Best,
Stefan

 

[jrfred453]

I've been slowly building a G9 and have been watching this thread closely. I've made all the purchases already except for the Lundahl transformers. Is there a Lundahl output transformer that is better suited for the G9 than the LL5402?

Also I'm curious how much chopping is needed to use a ECC88 tube? And getting 6.3v?

 

[1954U1]

Even with an OEP output, you'll still loose a couple of db. It seems that the ECC82 is really struggling to drive that transformer. That causes the slightly tilted freq response you can see in an earlier post.

Yes, there is still some bass loss [but less than a couple of dB], in my previous test case, using the OEP as output and the Carnhill as input..

to be more accurate, at 20Hz there is:
-0.98 dB with 40 Ohm input imp
-1.10 dB with 150 Ohm input imp
-1.13 dB with 600 Ohm input imp

At 30 Hz:
-0.79 dB with 40 Ohm input imp
-0.81 dB with 150 Ohm input imp
-0.84 dB with 600 Ohm input imp


And yes, these losses seems to coincide pretty well with these shown on pag 3 of this thread by bernbrue, which use a Beyer as input trans and an Edcor XS1100 as output,

I for now can live with -0.8dB at 30 Hz, but who knows, maybe me too I'll try the ECC88 mod..