Pre volume between EF86 and 12AU7

[Bender]

Hi everybody, glad to post there.

I'm planning to turn an old hi-fi mono tube amp into a guitar amp. This amp comes from the end of the 50's, it's a french (I am too) old brand called ALFAR ; it bought it for 30 bucks. There are one 5Y3, one EF86, one 12AU7 and two EL84.First I want to send directly the signal from the guitar into the EF86 control grid. Then I want to add a pre volume & the tone control between the two preamp tubes. But the anode from the EF86 is directly connected to grid of the first stage of the 12AU7 without coupling cap. So I don't feel comfortable with this, could it work either there is no coupling cap ?

Thanks a lot,

RG

[jjasilli]

The simplest thing is to plagiarize an EF86 circuit that you like.


Preamp pentodes tend to have very high plate = output impedances.  So it is good fro them to be buffered to drive the next stage.  Your existing circuit may be a cathode follower, or whatever, to buffer the output of the EF86. 

[birt]

what you have is perfect for an old AC15. check how different it is from that schematic and use it as a base for you changes. it's a great little amp.

[jjasilli]

Agreed. 


But Merlin wonders (Designing Tube Preamps for Guitar & Bass) why the EF86 is not put later in the chain -- this gives the option of overdriving the pentode with a prior gain stage. 


My guess is that the use of the EF86 in guitar amps was borrowed from hi-fi use, where a pentode first stage could give a strong boost to a weak input signal.  And overdrive was not a strong feature back then for guitar amps.

[kagliostro]

I presume your 12au7 is used as LTPI, if so and you want to use a Tone Stack after the pentode, you can easily add a mosfet

configured as Source Follower, Tubenit used this trick in many amps with good results, if you do a search on the forum you

can find more than an example, one other option will be to use triode of the 12au7 as Concertina PI and the other triode as

CF placed after the pentode and followed by the TS

Franco

[silverfox]

Is your amp one of these designs?

silverfox.

[HotBluePlates]

... First I want to send directly the signal from the guitar into the EF86 control grid. Then I want to add a pre volume & the tone control between the two preamp tubes. ...

I presume you want to use the EF86 because of its gain (for a single stage).

However, all preamp stages have a gain dependent on their plate load resistance. But that's not the end of the story... Any additional resistance to ground which follows the tube stage is effectively in parallel with the plate load and reduces the total effective resistance. And this lowers the gain of the preceding stage.

A volume control after the EF86 would be a resistance to ground after the EF86 stage, and a tone control circuit would also be an additional resistance to ground. Adding all this stuff would likely lower the EF86 stage gain significantly.

[silverfox]

One possible way to implement a volume control using the EF86 would be through the use of a Morph control and vary between Pentode and Triode mode. Merlins book has a design for that.

silverfox.

[kagliostro]

One possible solution to have some control on the signal



a table of configuration used with ef86 tube on various amps



---

About the use of a mosfet as Source Follower I was meaning something like this (V1b + IRF820), it is a simply add that didn't require

any filament current to be disposable and the consumption of B+ is very low



Mosfet Follies docet



Franco

[Bender]

Wow that's a lot of interesting responses, thanks guys!

I had an attempt to map the circuitry, it was a tough job (sorry for the poor quality of my photo). I'm trying to understand how I could transform it to be an AC15 based amp.



I am not used to fully design amps. Can someone highlights the bounds of the channel 1 on the AC15 scheme ? And I'm not sure to well understand the way to fit the mosfet onto the AC 15 scheme.

I added pictures of the beast, that is a sweet chassis to be a head amp, very robust.





Thanks,

RG

[kagliostro]

On your draw there is an arrow, indicating pin #9 of the ef86 tube, labeled Tone Stack ...... revise your draw and report please

pin #9 is G1, the input, in some old "HiFi" circuit you can find a tone control at the input, but not so often, so ... revise to be sure

---

R3 and (V)R4 are connected to ground at the junction ?

Franco

[Bender]

Hello Franco,

I drew this arrow to symbolise the coming signal, in the original circuit all the tone controls (volume, bass and treble) are curiously just after the audio input and before the EF86 ... Unless you want me to draw the original tone stack ?

NB.: R3 et R4 are not grounded at the junction

[sluckey]

You don't need a MOSFET to build an AC-15. All you need is EF86, 12AX7 (not 12AU7), two EL84s, and a 5Y3. Here's a link to my AC-15 Lite conversion. This can also be a Marshall 18 Watt Lite if you prefer.

Schematic link is at the bottom of this page...

     http://sluckeyamps.com/hammond/hammond.htm

[kagliostro]

No, not necessary to draw the original TS at the input, also, as Sluckey says, you don't require to add a Souce Follower if you want to copy the

AC15 circuit, to add the mosfet will be necessary only if you want to add a conventional ToneStack, not necesary if you adopt the AC15

rotary Tone Control

Franco

[PRR]

> attempt to map the circuitry

It is almost certainly a lot like the Mullard5-20 posted in Reply #5.

Work from that, figure out what is different.

However if you want a GUITAR amp, you are surely going to want something very different. The hi-fi amp takes a 1V signal and treats it very accurately. A guitar amp must take a 0.020V signal and beat it up.

Pentode and 12AU7 is probably not enough to do it all. Pentode and 12AX7 is more likely to do what you want. The power supply and EL84 connections can mostly be the same, but much of the small-tube circuitry needs a major do-over.

[birt]

oh and that donor amp look great :-)

[silverfox]

To me it looks like you're going to have to gut the chassis and start from scratch. Don't know if you considered that. However, the transformers and chassis will save you quite a bit of cash to start with. If you mod it as is,  and that looks like it would be a hassle, if you mod it without starting from scratch you could get it to work for a while only to have a catastrophic failure shortly thereafter. The other question would be: How would you add another tube socket, (gain stage) without stripping the chassis? This is like building a Hot Rod from an Antique Car. Fun but it's a project. Oh and you'll learn alot! I've built a similar "power amp" version based on the Dynaco and it works fantastic as a Pedal Amp. Merry Christmas,

silverfox.

[kagliostro]

I would like to see other photos of the amp without the cover


Franco

[Bender]

Hi folks, hope you had a great Christmas day.

Thanks y'all again for your replies.

I agree for a full re-building, moreover all the contacts are dirty and the components are antique, keeping them would be risky. At that time, I would like to have your support about the power section. Please find the scheme I made ; note that the tensions were taken with just the 5Y3 plugged.


I assume that it is not straightly suitable for the new circuit, isn't it ? Maybe few RC sections are required to fit it ?

Another question : what does the capacitor across the choke secondary and the power transformer ?

@kagliostro about a picture without cover : this is not possible without stripping all the front panel, what would you like to see accurately ? Please find another picture of the top.


Thanks,

RG

[silverfox]

You're off to see the Wizard. Don't know what your plans are at this point but, just in case, before you strip it, lets see if we can come up with the power supply. Before a power supply can be recommended you'll have to pick a design that will fit most of the hardware you have. The forum members will need to know your level of expertise. Are you able to work from schematics or will you need a layout? Another question that perhaps someone else already knows the answer to is: That should be an Ultra linear output transformer if it is a Dynaco or Mullard design but I only count 5 connections points, the transformer may have a lead grounded through the frame/chassis and other connections not seen.

Do you know what family of guitar amp you want to build? If not, what sort of tone or style of sound do want to get out of the amp?

Here is a link, on this site, to a build diagram for an AC30 that shows some of the information that is available if you haven't already discovered it: http://el34world.com/Hoffman/files/Hoffman_AC30.pdf. There are many styles to choose from here: http://el34world.com/Forum/index.php?board=15.0

silverfox.

[Bender]

Well, first about my skills. I built simple amps as 5F1, I modded other amps but never fully designed one. I would feel pretty much more comfortable with a layout.

About the sound I would like to reach, I want it to be overdrived with a sligh compression but still with a "clear background" as the Vibroverb does. I think that I have to keep the EF86 because it is pretty unusual, I heard about that it gives a nice compressed sound.

Thanks,

RG

[HotBluePlates]

I agree for a full re-building, moreover all the contacts are dirty and the components are antique, keeping them would be risky. At that time, I would like to have your support about the power section.

... lets see if we can come up with the power supply. Before a power supply can be recommended you'll have to pick a design that will fit most of the hardware you have. ...

99.9% of the time, the smartest move you can make is to leave the power transformer, power supply, output tube(s) and output transformer exactly as it already is.

This is because the characteristics of all those circuit elements are inter-dependent, and generally work best as they are already configured (because they were designed to go together as they are). Changing any of those in any significant manner usually means buying 2 new transformers to allow the change to be effective.

I'd suggest replacing any parts you think are marginal (usually old capacitors or possibly wires with brittle insulation), and focusing any changes on the preamp. You can do otherwise, but I believe you'll find (as I did) that if you mess with the power supply/output section, you'll either wind up spending enough to have built from scratch or will have underwhelming changes.

[kagliostro]

Bender I was just curious to see the disposition of the tubes and transformers

(thinking to the Vox AC10 that has only one tube more, one ecf82, respect your amp)



Franco

[HotBluePlates]

Another question : what does the capacitor across the choke secondary and the power transformer ?

Are you certain there is a cap connected across the choke as you've drawn? What value is it?

If it was a small value (on the order of 0.1uF) then the cap and choke would be a parallel-resonant circuit to strongly-reject a single frequency (and a band  on either side of that center-frequency). This might be centered on/near 100 or 120Hz to reduce the mains ripple in the rectifier output.

When this was done in the old days, it usually wasn't the only filtering present, but just a first-step before additional filter caps followed.

[Bender]

I think that Sluckey's drawing is the simpliest to make because of the same tubes used (please see attached).

According to HotBluePlates, I should keep the original circuitry and just mod the preamp unit. In other words, if I'm right, I have to copy Sluckey's scheme from the input jack to the connections toward the two EL84's pins #2. Nevertheless, my power supply doesn't deliver the same tensions than the ones coming into the EF86 and 12AX7 on the scheme. That is why I don't see how to copy the preamp section without changing a part of the power supply.

Thanks,

RG

[birt]

take those voltage measurements with the 5Y3 installed and without the other tubes. you'll get more accurate readings of what the voltages will be like. they will still be on the high side but you didn't mention if this was a working amp. if it is measure with all the tubes installed as well.


i think your B+ will be close to 375 before the filter caps with all the tubes installed.



i would just take good voltage readings, mark the wires of the PT and gut it. then i would build an AC15 and also copy the power supply using the transformer and choke you have there. these amps are simple and if the B+ is too high you can shave a bit off in different ways. for example with a GZ30 instead of 5Y3. this has pretty much the same characteristics and pinout but has a bigger voltage drop. (71V, for 5Y3 it's 60V and 5Y3GT 50V)


the AC10 that was mentioned is also pretty much the same amp but with an extra tube for a vibrato channel

[Bender]

Hi y'all,

It has been a moment since I posted. As a student, I can work on this project only during my vacations.

At this stage, my problem is to know if the original PWT is suitable for Sluckey's AC-15 circuit : http://el34world.com/Forum/index.php?action=dlattach;topic=19583.0;attach=55535

Please see the voltages I measured with the 5Y3 plugged : http://www.zupimages.net/viewer.php?id=15/52/7hkx.jpg

My problem is the following : is the PWT suitable ? Is there a way to fit the voltages ?

Sluckey thinks it is not. I ask if someone has an idea to keep me from buying a new $100 PWT.

Thanks, have a good day,

RG

[sluckey]

After re-reading this entire thread I'm gonna reverse my position about using your PT. Those voltages you listed are very high, but that's with no load (only the rectifier). Those voltages will drop when you plug in all the other tubes. And since your original amp uses EL84s the voltage will probably drop to a safe range.

So, I think your PT is suitable for an AC-15, or any other EL84 based amp. The OT is suitable also.

[Bender]

Thanks, that is a good news !

would you use the choke as in the original circuit ? I do not know a way to measure its inductance without a power generator,

Regards,

RG

[sluckey]

Yes, but I think your wiring diagram incorrectly shows how the choke is connected, not so much the choke but the cap across it.

[Bender]

Hi,

I gutted the amp so I can not check again, but I am almost sure that there was a capacitor at this place. It is a big electrolytic cap, the value is about 36 micro farads. Should I remove it in the new circuit ?

Thanks,

RG

[kagliostro]

Give a look to the photo of the circuit to verify

Probably the negative side of the 36uF electrolytic capacitor was connected to ground



I don't think an electrolytic capacitor was used for a Tuned Filter





Franco

[Bender]

Just have checked, the diagram is correct, the capacitor is at the right place. I took down the capa in order to read the hidden characteristics : 2x50 uF, 350/400 V.

[sluckey]

Both leads of the choke will be connected to that cap can. But there are TWO caps in that can. What you are missing id that the body of that can is connected to chassis ground. That's the common connection for both caps inside the can.

[Bender]

Thanks for you help guys ! I was perplex while reading those "2x50 uF".

So, what's the plan for this conversion ?

My opinion is :

-to keep the power section as set up in the original circuit as show on my PWT diagram as far as the choke

-to keep heaters connections

(I have another double cap, I don't know if it will be useful...)

-build the AC-10 or 15 circuit from the choke secondary

Question : if I want to build a termolo circuit as on AC-10, is the power transfo able to stand another tube ?

Thanks,

RG