Mini-AC30 based on Vox AC4

[Big_Mike]

I currently am an at-home player and use a couple of single-ended amps that provide plenty of power for my needs.  One of the amps is the Vox AC4-C1 (the blue one) with a 10 inch speaker.  This amp has the Vox top boost circuit only. 

At 4 watts or so, this amp has plenty of power for home use.  I do have some problems with the amp, and am contemplating doing a point-to-point version of this single ended amp.  The problems are as follows:

1.  The amp has a ton of gain on tap, more than I need. 
2.  The amp could use a very good output transformer.  The OT is a cheap one in this amp.
3.  Very little bass in the amp, very high treble (possibly related to the cheap OT)
4.  I have been told that the top boost circuit used in the current AC4 is not true to the JMI top boost from the AC30 that we know.  The new circuit in the AC4 appears to emphasize different frequencies than an AC30.
5.  The cabinet is small and cheap.  I want to make a nice sized 12 inch cabinet, or possibly make a head version of the amp.

My goal is to build a single-ended EL84 amp to sound as close to possible as a Vox AC30.  Master Volume along with a Cut control are a must. 

I am attaching a copy of the AC4-C1 schematic to this post.  I am not great at comparing schematics or making layouts.  Would anyone be able to determine if the top boost circuit is different from a 60s JMI AC30, and if so, how?  Also, if I could get some help on transitioning the attached schematic to a point-to-point layout, I would very much appreciate it.  Thanks! 

[PRR]

> Very little bass in the amp

C5 screams to be 0.01u or more. As-valued that's a 600Hz bass-cut.

R9 really should be 820r (Fender's fav) or less.

[Willabe]

Other than the R9 value on the cathode (K) of the cathode (CF) followers driver that drives the CF that PRR just wrote about, the top boost circuit is true/exact to the old Vox circuit.

(Yes, the treble cap is 47pF in your amp and the Vox had a 50pF cap there but no 1 can hear the difference in 3pF's. Besides those caps were 20%, 40pF to 60pF, tolerance anyway.)   

[Big_Mike]

So it sounds like the top boost circuit is basically the same, other than that one resistor.  I am curious what a turret board would look like for this amp.

[Willabe]

So it sounds like the top boost circuit is basically the same, other than that one resistor. 

No, Vox used the 1.5K value for the R9 in your schemo.

PRR was saying 'it should be' 820 ohm like Fender used. (He has a reason why he wrote that.)

If you scroll down to the bottom of this page, or any other page in this forum, there's a link for Doug's tube amp library of schematics. Compare them for your self. 

[John]

Before you dive into a turret board, I'd recommend the easy changes PRR has recommended. For C5, (if physically possible) just tack in a .022 across that 47pf. Same with the R9, parallel the 1.5K with another 1.5k, that'll get you close enough to see if you like the tone change.


To knock the gain down some on the first stage, parallel the 220k with another 220k.


Of course, if it's a PCB these might not be that easy to do. Just suggestions. :)

[Big_Mike]

Unfortunately, the AC4 is a PCB.  I'd like to make it into a point-to-point turret board amp to experiment more easily.  As I said in the first post, one of the problems with the amp is simply cheap Chinese parts, such as the OT.

[John]

Yes, I should have known it was PCB, pretty much everything is nowadays. Unless someone beats me to it, I'll work on a board layout for you in my spare time.

[sluckey]

It would be helpful to give some info about the present layout and size of the amp chassis. Maybe some pics with a ruler. Maybe a simple drawing showing tube line up and spacing, transformer placement and type, control panel layout, etc. Then a replacement board can be designed to fit that layout. It would be frustrating to design a 12" x 3-1/8" board only to find out later that there is only 10" x 2" space to put it in.

[John]

Hey, I never said it would actually fit.


 

[Big_Mike]

I will take some pictures and post them.  I plan to put this in a new cabinet or perhaps make it into a head, so don't feel bound by the dimensions of the stock cab.

[sluckey]

Cab doesn't matter. The chassis does matter if you will reuse it.

[drew]

http://www.voxshowroom.com/uk/amp/ac4c1bl_hood.html

If this is the amp we're talking about, it's not going to be simple to replace the PCB with a turret or eyelet board.  Sockets, pots and input jack are all on the board, and it looks like there is a very small area to work with.

[Big_Mike]

Yes, this is the amp.  You really cannot use the existing chassis because the PCB is so small.  I want to start over with a new chassis, but pictures of the existing one would probably help to create a turret board.  I will try and upload some pictures of the guts tomorrow.

[kagliostro]

Definitely this is the construction of a new amplifier, no modification of something existing


Keep your AC4 as is and go for a new build with new parts


Franco

[Big_Mike]

I agree, this would be a build of a new amplifier based upon the circuit of the current AC4-C1.  Point to point wiring, with additional features such as a cut control.  A much better OT will be needed as well.

[pompeiisneaks]

So it sounds like the top boost circuit is basically the same, other than that one resistor. 

No, Vox used the 1.5K value for the R9 in your schemo.

PRR was saying 'it should be' 820 ohm like Fender used. (He has a reason why he wrote that.)

If you scroll down to the bottom of this page, or any other page in this forum, there's a link for Doug's tube amp library of schematics. Compare them for your self. 

Willabe and/or PRR.  Why the 820 ohm Cathode Resistor?  Just biases it more 'hot' right?  Gives a bit more drive? 

If I understood the biasing of the cathode anyway.  The cleaner 12AX7's usually bias at 3.3k.  More often its 1.5k for a more balanced bias (some crunch but still pretty clean headroom) and anything lower is even hotter bias?  Or am I getting something all crossways?

~Phil

[Big_Mike]

Attached are some gut shots of the AC4-C1 PCB.  The first picture is the stock amp with no changes.  The second picture I got from the web where someone upgraded the output transformer and made a few other mods.  This should help to prepare a turret board.

[sluckey]

You'll probably want to mount the tube sockets on the turret board. Kinda like this...

[Big_Mike]

I was thinking about a chassis with the tubes hanging down, kind of like a Champ or a Deluxe.  It would allow for changing of tubes more easily. 

Another option would be something like a smaller AC15 chassis, with the tubes sitting upward on a tray below the turret board.

[Big_Mike]

In terms of turret board design, would it be possible to mod the Hoffman AC30 turret layout for a single ended EL84?  That way you could have a normal and top boost channel.

[sluckey]

Since you don't have any strict size limits on a chassis you can design the board and then pick a chassis that it will fit with a proper overall layout. I personally like to have a target size in mind for a chassis then design the board to fit. These are all 'loose' dimensions initially.

I highly recommend making up your mind about what circuit schematic you want. Then design the board layout to accommodate that schematic. You can certainly mix a normal and a top boost channel together to drive a single ended power amp. But get this worked out before thinking about a board layout. It's possible that Hoffman's AC-30 board just might easily adapt.

[sluckey]

This is how I would stich Hoffman's AC30 preamps to your AC4 power amp. Plan to experiment with the B+ rail resistors. C15 could be eliminated.

[Big_Mike]

This is how I would stich Hoffman's AC30 preamps to your AC4 power amp. Plan to experiment with the B+ rail resistors. C15 could be eliminated.

Thanks for the schematic, great work!  I will have to study it some more to see how the preamp and power amp sections connect together.  I will need some help translating the schematic into a turret layout.

[sluckey]

The junction of R19 and R20 is the output of Doug's preamp. C15 is the input to your power amp. One wire connects the two together.

[Big_Mike]

I see that now, Slucky.  I was trying to look at it on a small phone initially. 

[Big_Mike]

Slucky, pardon my newbie question, but on your spliced schematic above, would that have three preamp tubes?  I see the first preamp tube halves with each side handling the normal/top boost channel, the second preamp tube with the top boost tone stack, and then it looks like it runs into a third tube.  Is this correct?  Also, could you explain your comment about the B+ resistors and possibly having to experiment with them?  Thanks for your help!

[sluckey]

Yes, there are three preamp tubes. Half of the last one is not used. You may have to change the values of the B+ rail resistors. The values shown on your AC4 schematic will probably work but may not be optimum for an AC 30 preamp. All depends on your PT, but try the AC4 values first.

Are you confident that your AC4 schematic is correct? I'll post something more for you to think about later tonight.

[Big_Mike]

The schematic I posted of the AC4-C1 is a schematic that others have referenced on the web.  I have not done a complete analysis of the circuit, but folks who have modded the AC4-C1 seem to be in agreement that it is correct. 

[Big_Mike]

Wow, awesome Slucky.  Thanks for all the work on this, much appreciated!

[PRR]

> Why the 820 ohm Cathode Resistor? .... .... usually bias at 3.3k

3.3K with 100K plate resistor? An odd bias for 12AX7.

Take the Standard Fender Stage, 100K and 1.5K.

The plate will bias to about 70% of the supply voltage. For 300V, to about 210V.

Current in the 100K is about 90V/100K= 0.9mA.

Now direct-couple this to a Cathode Follower, as seen in 5F6a.

CF plate-cathode voltage is just 90V.

CF cathode current, for 100K cathode resistor, is 210V/100K= 2.1mA.

We are asking the second triode to conduct twice the current with half the plate-cathode voltage.

It does, but with hardly any reserve for up-swing.

However..... my sims are not showing a dramatic difference. (I don't think a 4:1 change in 2nd harmonic is a big diff in a guitar amp.)

Does someone have this setup and can quickly compare 470, 820, and 1.5K at HIGH level?

[sluckey]

If you decide to build this we would love to follow your progress. Even if you go some other direction we would still be interested.

[Big_Mike]

Sluckey, I will definitely post my progress on this project on the forum.  Thanks again for all your help.  One question I have is with regard to the right power and output transformers to use.  Edcor seems to have a good line of single ended output transformers.  Do you have a recommendation?

[sluckey]

The last SE amp I built was about '69. I don't have any recent knowledge or opinions about SE OTs. Several others here can give you some recommendations. I've heard good stuff about Edcor in general.

If you buy a good OT you may want to try it in your existing amp. Just may get you what you want without actually building a new amp.

[shooter]

SE OTs

I've used both Edcor and Hammonds line and they are well built quality OTs, but you pay for it.
My last build uses a Hammond 25W that I'm dumping about 34watts through and it just gets warm.
The *downside* if you want that *tweed* undersized OT "sound", you won't get it.

[Big_Mike]

Thanks for the replies.  Do you think a 10 watt or a 15 watt output transformer would be best, and are there any advantages to using a larger transformer versus a smaller one?  Also, Edcor has a ton of options.  Would a 3.5K impedence to 8 ohm be best?

[g-man]

Deleted

[okguitarman]

You might look at a PT-31 $13  If 5K/ 8 ohm would work.
That is what I just used in the backstar rebuild se EL84 PRR told me about them.

[Big_Mike]

What would be the impedence I would need for the OT?  There are so many options for some of the manufacturers.

[shooter]

impedence I would need for the OT

google-up el84 datasheet, look for the 1 tube section, Ra value.  typically it's based on your B+.
the sheet I looked at showed Ra about 5.2K for a plate of 250vdc

[kagliostro]

I open newly this thread for a question

I would like to know which is the AC voltage of the PT on the AC4TV amp

Many Thanks

Franco

[roseblood11]

Big Mike, have you build the amp in the meantime?

I'm planning something similar: Top Boost channel with a few switchable options, but with a cathodyne phase inverter and a push pull power amp based on the ECC99.

[jojokeo]

You might look at a PT-31 $13  If 5K/ 8 ohm would work.
That is what I just used in the backstar rebuild se EL84 PRR told me about them.

Who sells this and where? You should include this info since you are suggesting it.


I've used Matt's transformers like the one shown on the replacement and they are resonably priced. He has an online store "musical power supplies" and on ebay as mizicalmatt


I really like Magnetic Components aka Classic Tone (easy net search) also aside from the others mentioned. You can also get good prices with Weber's line and for single ended low power circuits its just not a hugely critical decision no matter which one you choose. No need to go with the extra cost and size by buying over rated and wattage OTs - don't believe the hype on other forums. One rated for your application is all you need.


Your issues currently are more likely part/value related and you should make all attempts to correct these things first. Coupling cap changes (make larger) and bias resistors & caps (helping with gain issues) should be the things to experiment with.

[Big_Mike]

Back on the forum after being off for a while.  I have not attempted this project.  I purchased a Vox AC10C1, but am finding the build quality cheap.  I am now interested in building a point to point AC15, without the tremelo circuit.

[sluckey]

Maybe my dual lite would interest you. It's a Vox AC-15 and Marshall 18W Lite in one small chassis. Very simple to omit the Marshall preamp if not needed.

     http://sluckeyamps.com/dual_lite/dual_lite.htm

[Big_Mike]

Thanks Slucky.  I was thinking of an AC15 just based off the Hoffman AC30 layout.  I would add a master volume to it, probably a ppimv.  How would you recommend altering the power tubes section on Doug's layout for two output tubes?

[sluckey]

You might look at a PT-31 $13  If 5K/ 8 ohm would work.
That is what I just used in the backstar rebuild se EL84 PRR told me about them.

Who sells this and where? You should include this info since you are suggesting it.

AES sells it.

https://www.tubesandmore.com/products/transformer-output-8-w-single-ended

[sluckey]

Thanks Slucky.  I was thinking of an AC15 just based off the Hoffman AC30 layout.  I would add a master volume to it, probably a ppimv.  How would you recommend altering the power tubes section on Doug's layout for two output tubes?

Doug's AC-30 is not an AC-15. If you want an AC-15 I don't recommend using Hoffman's AC-30 as a basis. However, if you want a Hoffman AC-30 that only runs two output tubes then build Hoffman's circuit as is. Then pull two tubes and change the cathode bias resistor to 130Ω. Should probably use different transformers too.

[roseblood11]

This is my version of a mini AC30. It has the LTP phase inverter and a ECC99 push/pull power amp. All parts values were taken from the early version AC30 with integrated top boost. This is the perfect amp for all living-room-the edge-wannabes... ;-)
The sound is much closer to an AC30, because it is NOT based on the AC4 - the phase inverter and power amp are very important for the sound.

[roseblood11]