AC30 build with spare parts

[Markusv]

Hi -Long time lurker here and I have built a Liverpool about 1 year ago

My question to anyone who is willing:
I have a beefy Nordmende PT from a console style Hi-Fi.
2ndaries are: 240V-0-240V; 6.3V; 7V and 3.15V / 3.15V
It used to drive multiple ax7's and 2 ell80's (which are really 2xel84's in a single bottle
So, it looks like a nice 30 watt build potentially

1. Without a 5V option for a rectifier tube, can I still approximate the AC30 vibe by using SS rectifier?
2. If so, do I still use a choke, or do I go Large resistor as in the Liverpool?
3. Does an AC30 really have to use an ef86 in the preamp vs the later 12ax7 versions? (In other words, is the sound that different?)
4. Do you think that the ell80's are OK in this application? Or should I go with the original 4xel84?

I want to just build the Normal channel and drive it with a treble booster for some grind if needed

Thanks for reading
I look forward to any insights

Regards,
Markus V

[RJ Guitars]

Mark,

I'll say up front I've got a lot of prejudice that taints my objectivity because I think the Trainwreck Rocket is probably the ultimate in an AC-30 type of amplifier.

Having said that:

The top boost channel on the AC-30 doesn't use the EF-86 so I say you don't need it. The Rocket is basically the same circuit as the Top boost channel from the AC-30.

The AC-30 and the Rocket use a GZ34 Rectifier tube, the stiffest of the tube rectifiers... leading me to say that with your slightly lower voltage, it works out just about right for the final B+ voltages and I don't think you'll ever miss that GZ-34. Paul Ruby built an awesome Rocket inspired amp that didn't have the Rectifier tube or the choke... you'll never miss them.

The only down side to using the EL80's would be when they go junk you'll have a hard time finding them... they should function just fine. On the other hand EL-84's are in production and quite abundant, so it's a matter of how conventional you want to be.

If you look on the Trainwreck pages you'll find a zillion posts that sing the blessings of the Rocket amp... it should at the very least inspire you.

Let us know what you build and if you have any questions I'd be glad to help out... I'm a bit Rocket crazy! I've put together a few for myself and a couple dozen "Roc-Kits" for others - awesome amps!

rj

[Phil_S]

Your 240-0-240 will give you B+ ~312vdc with a 6CA4 rectifier. 7A for filament current is well over what you need to power all the tubes. A full wave bridge will give you B+ ~336VDC and it will be rather stiff. If you are one of those players who knows how to milk the sag for tone, then you want the tube rectifier. If not, it doesn't matter much which you choose.

A big resistor is not the same as a choke. A choke has inductance, which removes AC ripple from the B+ supply. A big resistor is a simple voltage divider (reducer in this case). You need the choke to get the vibe. Even a smaller choke, maybe 5H-10H instead of the 15H would be better than no choke. You can buy a choke for around $20, just not the AC30 choke. Get one that is rated for enough mA. I'd guess 4xEL84 needs 120mA rating. Maybe you scout eBay for a cheap one?

If you have those ELL80's why not? Discard them if they are old and tired. This does happen eventually and it can't be reversed.

[Markusv]

RJ

Thanks for the reply
So here are my thoughts

1. I just found a schematic at Hoffmann for an AC 15 that uses a 5Y3 but 5K resistor instead of a choke
2. With my lower voltages and all I can then use ss rect. with a 5K-ish wirewound 20watt resistor like the Liverpool or Express - removing the Rectifier /choke issue. I do believe from some of the posts that the big resistor can induce some sag and compression, so as to somewhat mimic the effects of a tube rectifier (not sure my technical expressions are correct)
3. I have heard the rocket is an absolutely must-have and has great bottom end - perhaps slightly higher gain than AC30
4. If I go with this I may still build it as a combo with basic AC30 / 15 looks and cab design

I am curious as to whether the Rocket has as much noise an layout sensitivity as the Liverpool or express? I had unbelieveable hiss in my 'pool initially and tamed it over time


I am sure I will still ask questions in the future and I appreciate your offer of help

Regards,
Markus V

[Markusv]

RJ

Thanks for the reply
So here are my thoughts

1. I just found a schematic at Hoffmann for an AC 15 that uses a 5Y3 but 5K resistor instead of a choke
2. With my lower voltages and all I can then use ss rect. with a 5K-ish wirewound 20watt resistor like the Liverpool or Express - removing the Rectifier /choke issue. I do believe from some of the posts that the big resistor can induce some sag and compression, so as to somewhat mimic the effects of a tube rectifier (not sure my technical expressions are correct)
3. I have heard the rocket is an absolutely must-have and has great bottom end - perhaps slightly higher gain than AC30
4. If I go with this I may still build it as a combo with basic AC30 / 15 looks and cab design

I am curious as to whether the Rocket has as much noise an layout sensitivity as the Liverpool or express? I had unbelieveable hiss in my 'pool initially and tamed it over time


I am sure I will still ask questions in the future and I appreciate your offer of help

Regards,
Markus V

[Ampsterdoom]

A big resistor is not the same as a choke. A choke has inductance, which removes AC ripple from the B+ supply. A big resistor is a simple voltage divider (reducer in this case). You need the choke to get the vibe. Even a smaller choke, maybe 5H-10H instead of the 15H would be better than no choke.

Your mileage may vary - You need to build one with a choke and one without and A/B them to be sure because the ears are the ultimate tone discriminators. My belief is that the difference will be "in the noise" so to speak.

http://www.aikenamps.com/Chokes.html

A good piece of work on chokes. The placement of the choke in these amps places it in the "increased filtering" type of use.

The AC-30 Transformer is 280-0-280 and the Rocket is 250-0-250.

The AC-30 choke is 100mA and 15H. a 75mA 15H (Hammond 158L) has been used in several Rocket clone amps.

Doom

[Markusv]

Thanks for the replies!

I will probably go with the choke then.
Just one curious question: why do the other TW amps get away without a choke and the ripple does not seem to be an issue?

Markus V

[RJ Guitars]

So here are my thoughts

1. I just found a schematic at Hoffmann for an AC 15 that uses a 5Y3 but 5K resistor instead of a choke
2. With my lower voltages and all I can then use ss rect. with a 5K-ish wirewound 20watt resistor like the Liverpool or Express - removing the Rectifier /choke issue. I do believe from some of the posts that the big resistor can induce some sag and compression, so as to somewhat mimic the effects of a tube rectifier (not sure my technical expressions are correct)
3. I have heard the rocket is an absolutely must-have and has great bottom end - perhaps slightly higher gain than AC30
4. If I go with this I may still build it as a combo with basic AC30 / 15 looks and cab design

I am curious as to whether the Rocket has as much noise an layout sensitivity as the Liverpool or express? I had unbelieveable hiss in my 'pool initially and tamed it over time


I am sure I will still ask questions in the future and I appreciate your offer of help

Regards,
Markus V

Markus - the talk about chokes and the AC-30 has been kicked around for a long time. If the AC-30 is a class A amp (I don't think it is), then there is no sag - thus the choke is only to smooth out the AC ripple. Depending upon how you bias your EL-84's, you could get some class A/B action and then sag is possible. I tend to think that you need to have an amp working really hard before you get the phenomenon of sag. So I won't say the choke doesn't do anything, but I'll probably stick to my story that you'll never miss it - especially if you don't run this thing cranked all the time.

Some folks use a big inductive resistor and refer to it as the "SAG" resistor. I think it probably works that way and is worth adding in there.

Just today I put a half dozen Rocket power transformers into inventory. I was looking at them and they say 200/250 on them. Has me wondering if maybe at least one iteration of the Rocket used a 200 V AC secondary... I've studied that amp for a couple years and never run across that before but I never had an original spec transformer in my hands before either.

The Rocket has almost exactly the same gain structure as the AC-30... same tubes and almost exactly the same components in the same order.

The Rocket does have a very different gain structure than your Liverpool and it is much more friendly as far as lead dress and such. The Rocket uses one less gain stage - a 12AX7 has a gain of 100 so take whatever noise problems you've got and multiply them by 100 for a Liverpool or Express... the Rocket is much more kind.

I like the idea of a combo, although Trainwreck never built one that way. I plan to do it for myself and think the Rocket would be excellent as a combo amp.

Anything you need, lemme know. As I said, there are currently a ton of posts on this amp and you can learn from all our mistakes and successes without the pain if you want to.

rj

[Markusv]

RJ

Thanks - I appreciate the time spent to deal with each of my questions.
I will follow up for sure when I get all the ducks in a row.
I may need to buy an OT still as I think I need beefier stuff than what the Donor amp PT was originally equipped with

Regards,
Markus V

[RJ Guitars]

...I may need to buy an OT still as I think I need beefier stuff than what the Donor amp PT was originally equipped with

Regards,
Markus V

Markus - your timing for this point in history is pretty good since the Rocket has been studied pretty hard this past year and we believe we know the exact output transformer used... a variant of the famous Dynaco 470 output tranny specially wound by the Heyboer transformer company.

The rest of the story on the good timing is that I just ordered a dozen of these that I am passing along to the forum members at cost... see this thread - http://ampgarage.com/forum/viewtopic.php?p=66624#66624

I think I have 6 left if you are interested in one of them.

rj

[Markusv]

Thanks, RJ

I think I may have access to something close to the dynaco OT's .
I want to do this as a project with things that are kicking around on my bech and in my workshop.
If it even remotely a success, I will go for the kill with all spec'd components

Markus V

[Phil_S]

Just one curious question: why do the other TW amps get away without a choke and the ripple does not seem to be an issue?
Markus V

The TW has a rather massive bank of filter caps and uses a solid state rectifier. The AC30 has neither of these. With a tube rectifier, you are limited in how much capacitance you can apply to the circuit before you get above the limit that's applicable to the rectifier tube.

[Markusv]

I see that in the AC30, yes.
16/16/8/8/uf

But in the Rocket schematic, it is
40/40/choke/40/20/20/20/. Is that not as heavy a fliter in terms of capacitance as any other wreck?

Markus V

[RJ Guitars]

I see that in the AC30, yes.
16/16/8/8/uf

But in the Rocket schematic, it is
40/40/choke/40/20/20/20/. Is that not as heavy a fliter in terms of capacitance as any other wreck?

Markus V

Mark look at that first set of 40uf/40uF - they are parallel so that's 80uF. Now if you were to put your standby switch in between them, then they would not smack that tube rectifier quite as hard when you light up the amp... look carefully at a proper Rocker schematic and you'll see how that is done. The Rocket power supply is similar to the Liverpool and Express except for the tube rectifier.

Ken Fischer liked stiff power supplies and preferred the GZ-34 because it was the stiffest of the Rectifier tubes.... you'll find that in "The Train Wreck Pages" of Gerald Weber's book.

With the GZ-34 I drop about 17 volts of B+ on my Rocket... you can use a 5U4GB if you want to drop a little more or put in one of those solid state rectifier plugs if you want to max out the voltage. With your 240V power transformer you'll stay within the specs the tubes can handle. You'll hear a bit of difference in all of them.

Look for a HiFi quality output tranny - equivalent to the Dynaco if you want to capture the Rocket vibe. I built a Rocket using an AC-30 output and they do sound a bit different.

good luck and let us know how it's going,

rj

[Markusv]

I see that in the AC30, yes.
16/16/8/8/uf

But in the Rocket schematic, it is
40/40/choke/40/20/20/20/. Is that not as heavy a fliter in terms of capacitance as any other wreck?

Markus V

Mark look at that first set of 40uf/40uF - they are parallel so that's 80uF. Now if you were to put your standby switch in between them, then they would not smack that tube rectifier quite as hard when you light up the amp... look carefully at a proper Rocker schematic and you'll see how that is done.

** I have the Taylor Schematic Version AO. Is that right?
...So back to my original issues with the PT that has no 5V heater taps... If I did go with the SS rectifier, can I simply use that as if in a liverpool and do the rest of the line-up of Cap filtering and choke?

Look for a HiFi quality output tranny - equivalent to the Dynaco if you want to capture the Rocket vibe.

**So that would typically be something with ultralinear taps that I just insulate and tuck away?



**Thanks a great deal for your help

Regards,
Markus V