jcm900 preamp

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
Post Reply
User avatar
carlozsulca12
Information
Posts: 16
Joined: 05 Nov 2020, 11:53
Has thanked: 1 time

Post by carlozsulca12 »

hello, i got some time so i'm back with the pedals and recently i got some 12ax7 and a little bit bored with silicon, i started building preamps of recognized amps, i built the preamp of a jcm800 and all good, so now i want to build the jcm900 preamp, but I have a doubt about the signal input and the behavior/function of the marked line, I hope that someone who has some experience in this can help me
Attachments
Captura de pantalla 2022-10-22 080831.png

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1671 times
Been thanked: 1344 times

Post by Manfred »

This is a connection to the normally closed contact of the input jack.
If no instrument cable is plugged in, the grid of V1b is also connected to ground.
This is obviously to prevent noise or oscillation in the open state of the jack.

User avatar
bmxguitarsbmx
Cap Cooler
Information
Posts: 510
Joined: 25 Mar 2010, 21:15
Has thanked: 704 times
Been thanked: 216 times

Post by bmxguitarsbmx »

Go play a jcm900 before you put a bunch of work into it. I built a jcm900 some decades ago because it was presumably the highest gain circuit in the Aspen Pittman book. It was awful. I hated it. I was convinced I did something wrong. Spent forever swapping ceramic for silver mica and carbon comp for metal film. To no avail! For some stupid reason I ended up spending all my money on a jcm900 and it was at that moment I realized that I was an idiot. Still hate that amp. Still sorry to my buddy Tim, who bought it for $350. That was precisely the moment everything went wrong for him.

My 2 cents

User avatar
Umulamahri
Information
Posts: 2
Joined: 25 Nov 2008, 19:13
Been thanked: 1 time

Post by Umulamahri »

SL-X is much better.

User avatar
Cub
Cap Cooler
Information
Posts: 458
Joined: 29 Aug 2011, 07:14
Location: The Netherlands
Has thanked: 102 times
Been thanked: 118 times

Post by Cub »

The SL-X used to be my favourite of the JCM900 series and its specs certainly are attractive. Gain from 0 to 10 and then you can dial in an extra 12AX7 from 10 to 20 with the sensitivity pot for even more gain, what's not to like ?
But then I got to compare my 2100 SL-X with an earlier Master Volume model that uses a W005F bridge rectifier and an added 1N4007 instead of an extra tube. The lows were much tighter on the earlier amp and so much less hiss. I almost felt a bit cheated.

I always wondered if it would be possible to add a switch to it so the clipping from the rhythm and lead channels of the Silver Jubilee would be options as well. Carlos, that might be a fun little experiment for you to try !
Or would it be better to do it the other way around, a Silver Jub preamp with an added option for the W005F + 1N4007 clipping ?

On a side note, that bridge + 1 trick also works like a charm in a Boss DS-1 Distortion.
I wish I were a chestnut tree, nourished by the sun.
With twigs and leaves and branches and conkers by the ton.

User avatar
bmxguitarsbmx
Cap Cooler
Information
Posts: 510
Joined: 25 Mar 2010, 21:15
Has thanked: 704 times
Been thanked: 216 times

Post by bmxguitarsbmx »

Now the Silver Jubilee I like

Post Reply