V-241 from scratch

[opacheco]

It's RS tin plated equipment wire in various colours, .

Dave

Thanks DaveP

Do you have a source for them??

Opacheco

 

[DaveP]

As I said, it's RS a UK company.

http://export.rsdelivers.com/

But you may find it easier to order from the US or Mexico if these are difficult to source in Honduras

DaveP

 

[opacheco]

As I said, it's RS a UK company.

http://export.rsdelivers.com/

But you may find it easier to order from the US or Mexico if these are difficult to source in Honduras

DaveP

Sure thanks!

Opacheco,

 

[opacheco]

Well spotted Guavatone,

The reason for these small changes is because I have incorporated part of the V76 input circuit into the V241.

The published noise for the V241 is about -69dB whereas the V76 is around -118 to -120dB depending on the weighting used.

Remember that the OPT is completely different as is its inductance so I have used C9 (from the V76) to avoid interaction with the self resonance of the OPT and C10.

C2 is required as a by-pass cap for the cathode of V1, this varies the gain simultaneously with the feedback.  If the gain is varied solely by feedback then it would bring the amp to oscillation on the lowest gain settings, C2 helps reduce the gain so less feedback is required.

As far as I can see, R6 and C4 are configured exactly the same as the original, just drawn differently.

What I found necessary was to remove some top end feedback via C6, R9 and C7 from the mid point of the DC feedback.  This part of the circuit was necessary for the Edcor OPT I used for the original rest, it may not be necessary for the Jensen OPT.

The reason I like the V241 is the output stage.  It uses both sections of the ECC85 paired, this maintains the mu at 50, whilst halving the rp.  The very low rp allows the use of a simple 150H choke that is still available today.  The west German V series amps needed 400 to 700H chokes because the rp of the EF804 is so much higher.

As you noticed, the feedback of the original circuit was complicated because they needed to put caps on the feedback resistors.  This type of modification is sometimes required to control instability as it cuts the response over 20 to 30kHz as you have said.  Its unlikely that only one person made the V241, so slight differences in wiring from other technicians may have changed the capacitances in the layout and made these mods necessary, I know that Fender did much the same.

DaveP

DaveP,

Fantastic analysis!

How about the R23, R24, R25 in the original Schematics??, what will be the reason for are there??.....you don't use them, why?

Opacheco

 

[DaveP]

How about the R23, R24, R25 in the original Schematics??, what will be the reason for are there??.....you don't use them, why?

The original schematic suffered from hum, so they added a small amount to the feedback circuit to cancel it.
I use toroidal transformers and DC heating for the tubes so there will be less hum than the original circuit.

The original V241 was built as a  compact box (probably to save space on the desk) my version is built for rack mounting so it gives me more space to separate the power supply from the input signal.

DaveP

 

[opacheco]

The original schematic suffered from hum, so they added a small amount to the feedback circuit to cancel it.
I use toroidal transformers and DC heating for the tubes so there will be less hum than the original circuit.

The original V241 was built as a  compact box (probably to save space on the desk) my version is built for rack mounting so it gives me more space to separate the power supply from the input signal.

DaveP

Really fantastic!, thanks DaveP

Opacheco

 

[hop.sing]

Hi

I am following this thread with interest.
Since I have a nice enclosure, a proper power transformer and an unused sennheiser tmb 103  1:15 ratio  input transformer already, I think I could tackle that preamp.
Wouldn't an ecc81 be OK as a replacement for the ecc85? The tube datasheet looks very similar, if not identical except for the heater current. I have a few of those NOS  and it would be a shame not to use one of those.
Thanks  Tobias

 

[DaveP]

Wouldn't an ecc81 be OK as a replacement for the ecc85?

The short answer is that it might, but it might not have the same bass response.

The way to check is like this:

Copy the plate charts for the ECC81 and 85,  then, remembering that they are using paired tubes, (so you need half the current) find where 3mA at 250V is and mark it on each chart.

Draw the rp line through those points and compare, my guess is that the 85 will be around 11k and the 81 around 15k.  This means that with paired tubes you will have 5.5k for the 85 and 7.5k for the 81.

7.5/5.5 = 1.36 so your -3dB bass cut-off will be 36% higher frequency, but don't take my word for it, check it out

While you're at it, you can see if the mu is the same or higher with the 81.  The stated valve data are only a rough guide, only the data at the actual operating point will be good enough to decide which is best suited for the job.

DaveP

 

[hop.sing]

Thanks for your reply, calculating loadlines with an anode choke is a bit of a mystery for me, but replacing the choke with a resistor shows me that the Ri of both tubes should be around 20Kohm with 3mA and 250V at the OP. Maybe one or two Kohm less with the ecc85. 
So with an inductance of 150H -3dB is around 10HZ with both Tubes. Or 10Hz with ECC85 and 13Hz with the ecc81.
If I want to play save, I could use two chokes in series....
I think I can try the ecc81, seems close enough for me.
But:
Where is my mistake? You guessed 11k and I calculated nearly double the resistance. Does a choke make all the difference?

 

[DaveP]

You don't need to draw a load line as such because as the DC resistance is around 3k, the loadline is almost vertical.  So you can simply mark that point at 3mA and 250V.

There will be lines already on the chart that you can draw a tangent line on near that point, if it falls between two tangents then you can interpolate between the two.

It's simpler than you think   Plenty of sources on the internet for a primer.

DaveP

 

[leadbreath]

Legendary Dave legendary...!

 

[Alm6mountain]

Dave!
Another amazing project! I am fairly new to the tube preamp game and I wanted to see if my math on the power transformer secondaries checks out. I am planning on going through with PSU Designer II to triple check myself.

After the diode bridge on the plate you have 355VDC. Using the diode bridge formula (Vac = Vdc x 0.71) Gives me ~250VAC. This correct?

Also, for the DC filament section...Are you using 6VAC through the full wave diode config?

 

[DaveP]

After the diode bridge on the plate you have 355VDC. Using the diode bridge formula (Vac = Vdc x 0.71) Gives me ~250VAC. This correct?

Yes but this formula does not take into account regulation.

Power transformers are rated at maximum safe output (say 25VA).  If you take less than this, then the voltage rises.

In this project using this isolation PT, its output would be 240V if drawing the full 25VA, but as you have said, its more like 255Vac because I'm drawing less than the full output.

Also, for the DC filament section...Are you using 6VAC through the full wave diode config?

This is another example of the principle described above.  For tube heater supplies, it's better to use two diodes centre-tapped so you only have to worry about two diode drops instead of four with a bridge.  The heater TX is over rated so that I get more than 12V with  series connection, say 12.6V x 0.71 = 8.95V.

For this circuit to have minimum ripple, you need to be able to drop several volts between big capacitors.  I always make a rough birds nest circuit before the actual project to determine the value of the dropper resistor to get to 6.3V.  A 25W Al clad gets the job done.  For some reason 25W types are cheaper than 10 or 15W types.

DaveP

 

[mjrippe]

A 25W Al clad gets the job done.  For some reason 25W types are cheaper than 10 or 15W types.

DaveP

And the 25w will never even get warm, so it should last forever!

 

[DaveP]

Finally got some quality time today to join the front to the back and finish the wiring.

These are the tubes ready to go in.  The Edicron EF86 is the same tube that I used for the pentode testing on a previous thread.
The ECC865 is a special quality ECC85 with gold plated pins.

All that remains is to power it up to check the voltages and fit the covers, I will probably drill some vent holes over the tubes.

I will pass some audio to check all is in order and attempt the Signal to noise ratio and the gain check.  Then it will be sent to the UK for testing on the bench and if OK, in the studio.

I hope you've enjoyed the journey.

DaveP

 

[dmp]

Nice looking ground buss!
You are only connecting to the chassis at the input XLR?

Looks like you increased some cap values like C8.
Did you keep all the blocking caps at the schematic values (C3, C5, C9) ?

 

[DaveP]

You are only connecting to the chassis at the input XLR?

The earth/ground at the IEC socket goes direct to chassis as is the regulation I believe.  The amp earth bussbar is only connected to chassis at the input socket (Merlin's design).  The amp earth does not touch the chassis anywhere else.

I had to increase the output cap from 8uF to 10 because I couldn't source the lower value in that quality.

All the other caps are as the schematic except there is no cap on the input grid as I am using a V76 front end.

I will post the final schematic later as I may need to do some tweaking after testing.

DaveP

 

[dmp]

The earth/ground at the IEC socket goes direct to chassis as is the regulation I believe.  The amp earth bussbar is only connected to chassis at the input socket (Merlin's design).  The amp earth does not touch the chassis anywhere else.

I had gotten the ground scheme from looking at the schematic. In most schematics, including this one, the ground attachment point is at the base of the input transformer (v241, v76, BA2)

Is Merlin Blencowe who you are referring to?  Is this the book?
"Designing Tube Preamps for Guitar and Bass"
I would like a good recommendation for a book on building PTP amps.

 

[DaveP]

Is Merlin Blencowe who you are referring to?  Is this the book?

This is what you are looking for.

http://www.valvewizard.co.uk/Grounding.html

DaveP

 

[DaveP]

I checked the voltages and ran some sine waves through tonight.

This is the latest schematic so that you can follow what I've done.

The voltages are spot on and the amp puts out 14.8 Vrms before clipping at all gain positions into a 10k load.

When I built the rough birds nest version, I noticed that the top end started to fall off slightly so I added R9, C6 & C7.

This loses some treble feedback so the frequency response is flat to 20kHz at 50 & 55dB gain.

To put this in perspective, at 20dB gain the output at 20kHz is 1.37dB below the level at 1kHz.

At 30dB its -1.1dB and at 40dB gain it's -0.6dB.

I thought this was acceptable and gives the option of a subtle change of colour with lower gain values.

Back in January, Guavatone asked why I used  C9 for the feedback instead of coming off the OPT side of C10.

Well I tried that tonight and there was an oscillation at about 1Hz, so I decided to leave well alone.

I will run a few more tests before I call it a day and ship it off to Ian for him to put it through its paces.

DaveP