Parallel effects loop for Marshall style amps

[novosibir]

Referring to this thread

http://forum.metroamp.com/viewtopic.php" onclick="window.open(this.href);return false; ... c&start=15

I proudly can tell you now, that it's done

Until the end of the last year I never have engaged myself to schematic design programs, because it's always been so less time, and since my beginning with tube amps back in 1986 I've always drawn all my schematics and layouts by hand - these are some hundrets meanwhile.

So in the past I never could post any of my schematics here on the board, to help others, to give advices or only to lead anybody to the right idea with it.

The last few weeks every day I've been fighting with four different schematic programs - and finally I won I've decided now for 'ExpressSch' (a freeware), where I can create my own schematic symbols, which are sometimes quite different from the generally used ones, but you'd understand it easily!

In the other thread I've promised you a schematic about a loop, which is 'really' working fine and works with stomp boxes as well as with high class studio equipment. A loop, which has cost me a long time of developement in the 90' - but yes, it is working much better than anything else you know yet even from the amps of the 'big four'!

Although I don't intend to post it openly at the Metroboard, so that not everyone of those thousands of 'not members' simply may download it...

... but I'll send it to each 'active' Metro member by request for his personal use.

Yes - it's for Metro board members only!

The only I'm expecting from you is:

- a Metroamp membership of at least 3 months
- a minimum post count of 10
- that you keep it for your personal use
- and that you don't forward the schematic to anybody else

I'd be seriously pissed, if I'd find 'my loop' anywhen in a few years in other amplifier of professional production - and this even 'for free'! Hope, you'd understand

Keep in mind, that I can't PM a .pdf - hence your email is required in your request!

If you'd have any further questions concerning this loop, so please don't PM or email me, but don't hesitate to post your quest here in this thread, because your quest (and my reply to it) might be of common interest.

Larry

[Freebird]

Thanks a lot Larry!

I have a couple of questions about your fantastic drawn schematic:

a) Do you recommend insulated or non-insulated jacks for send/return against humming? Pro and con?

b) What type of is the added tube, a 12AX7?

c) There is little room in a JTM45. Where should I put in the additional tube in a already completed JTM45, on top or inside of the chassis and at which position?

d) Is wattage of all extra needed resistors 1 watt? Especially the resistor 4k7 which is connected to the supply volatge (Uc).

I'm looking forward to hear from you.

Daniel

[novosibir]

a) Do you recommend insulated or non-insulated jacks for send/return against humming? Pro and con?

By any means insulated jacks, because they usually will be located close to the OT secondary ground - otherwise with non insulated jacks it might occur a 'put-put' like ground oscillation. Also the tips of the jacks have to be grounded when nothing is plugged in (switching jacks)!

b) What type of is the added tube, a 12AX7?

Sorry, forgot to denote it in the schem! The tube is a 12AX7/ECC83 - but you also can try a 12AT7/ECC81 for a slightly cleaner and brighter sound colouration, although in this case the 68K resistor after the both 220K mixer resistor has to be increased to 100K, to balance the lower gain of the 12AT7.

c) There is little room in a JTM45. Where should I put in the additional tube in a already completed JTM45, on top or inside of the chassis and at which position?

In the small JTM45 chassis that's indeed a problem! You might place it between V2 & V3, but this solution I don't like too much. I've done it different and have placed an additional tube socket horicontal onto the small side of the chassis, what then became V1, V1 became V2, V2 became V3 (the loop tube), and V3 became V4 (PI).
I'll try to scan some pixs and upload it to my server this evening (yes, it's already evening here in Germany) and after that I'll post the links to it here.

d) Is wattage of all extra needed resistors 1 watt? Especially the resistor 4k7 which is connected to the supply volatge (Uc).

For all resistors 1/2 watt is ok except the 4K7, this should be a 1W.

Larry

[novosibir]

Update to my upper post:

When you're using a 12AT7/ECC81 tube instead of the recommended 12AX7/ECC83 tube for the loop section, then as already mentioned above increase the 68K resistor after both the 220K mixer resistors to 100K, to balance the lower gain of the 12AT7, but additional:

- decrease the CF's 1K cathode resistor to 560 ohms, and

- decrease the CF's 47K cathode resistor to 22K

The output impedance of the SEND then is about 200 ohms, much lower that the min. requirement of the studio cracks of 600 ohms.

Then you even could hook a 100 ft. long cable on the send w/o any audible high losses

Larry

BTW: The SEND impedance with a 12AX7 is about 900 ohms, low enough to hook a 30 ft. long cable on the send w/o any audible high losses!

[TomGibbs]

thanks very much for the schem Larry!

but im quite new to amp electronics and whereas some of your symbols are the same, some are quite different,

i know where the resistors are
and the caps
but is the circle with an arrow through it a potentiometer?
also, i havnt got teh schematic for the amp im building yet so im not sure, but how many new pre amp tubes are there? are the rest the existing tubes?

and last one now
in the middle, there is the send and return jacks, but what is the line for underneath them?

Thanks again !!!
Tom

[novosibir]

but is the circle with an arrow through it a potentiometer?

Yes, that's my pot symbol! The middle contact is the wiper, the upper contact always the pot's input lug.

... so im not sure, but how many new pre amp tubes are there? are the rest the existing tubes?

Yes! Left in the schematic you see the orig. V2 with the orig. tone stack. The loop section begins after the treble's wiper. And right you see the driver stage with the tube denoted 'V4', because I've denoted the loop's tube with 'V3'.

in the middle, there is the send and return jacks, but what is the line for underneath them?

These are two lines, one facing to left, one to right, and ending with an arrow. The arrow indicates the switching contact in the jack, which sets the tip contact of the jack to ground, when nothing is plugged in.

Larry

[novosibir]

And yes, to understand my jack's symbol correctly:

There are two concentric circles. The inner circle is the tip's contact (hot), the outer circle is the shield's contact (to ground).

The arrow you can see (which is usually grounded) is facing to that wire, which is going to the tip's contact of the jack and indicates the switching contact in the jack.

Larry

[novosibir]

There is little room in a JTM45. Where should I put in the additional tube in a already completed JTM45, on top or inside of the chassis and at which position?

Yes, that's true! Simply no place anymore on top of the chassis for an additional tube. The same problem I've had with some of my early JTM50 rebuildts anywhen in the late 80' - and I've had an idea, which you can see on the following photos:

http://www.larry-amplification.de/ampre ... h.89.1.jpg

http://www.larry-amplification.de/ampre ... h.89.3.jpg

Sorry for the blurry photos, but at that time I've only had a cheap-o pocket cam

Larry

[antosimoni]

Larry, sorry , but do you have any photo of the Loop ???

also can I use a 20u/450V instead of the 22 one??

PS : what does Uc and Ud mean???

...and (last but not least) YOU'RE GREAT !!!!!!!!

[novosibir]

Larry, sorry , but do you have any photo of the Loop ???

Sorry, no! I've seldom buildt it into a stock Marshall. Mostly it's been 2-channel or 3-channel Marshall rebuildts with a completely new layout inside, where the loop was included. So pixs wouldn't help much anyway.

also can I use a 20u/450V instead of the 22 one??

Shure, all values from 16

[thinlizzy]

Larry thanks for the schematic.

Is this loop easy to rebuild to a serial loop?
I guess removing 820K will get me there?

Thanks!

[novosibir]

Is this loop easy to rebuild to a serial loop?
I guess removing 820K will get me there?

Right! Simply place a break/make switch anywhere in the line of the 820K resistor, then in the 'break' position it's a serial loop!

Still 'more' correct would be, to use a DPDT switch and bypass the resistor after the Return Level's wiper with an additional 220p with one half of the switch, while the other half is breaking the line of the 820K.

The reason is, because every resistor network is damping especially highs, therefore the 220p is in the diagram as a 'bright switch' for the parallel path. But switched to serial, the path after the CF is the only remaining path - and now needs this bright switch too

Larry

[thinlizzy]

Oke I see Larry

Do we still need the 220k, 220p 110k and 68k if I want to make it a "serial only" loop? And will the loop still function the same?
I wanne put it on a small board as possible so I dont want any extra resistors that has no function.

Great you sharing this with us!!

[novosibir]

If you want to make a 'serial only' loop, you can skip the entire parallel row, but then decrease the 68K to 47K, to keep the internal levels correct.

But tell me! Do you really want to kill the origin sound of your amp? Do you really want to rob its entire dynamics range? You know, what you're intending? The entire preamp's signal pressed through semiconductors, and then fed back into the power stage again

Larry

[thinlizzy]

Good point there.

I want to use the loop to boost my volume for solo's. I want to put a boss GE-7 box in the loop. When I use it in parallel will I still get a good boost from this loop?

Marcel