Author Topic: Biasing a valve amp (Read 17771 times)

PUMPINGDELUXE · « **on:** July 09, 2005, 01:54:45 AM »

Just wondering how hard it is to bais a marshall valve amp,, a TSL602 Combo is the amp in question,
My local guitar tech quoted 60 quid excluding vales !

(as I have the a new set of valves already) ,, and 80 for a service,,,
To me that seems a bit harsh !!! or is that bang on the money ?

MDV · « **Reply #1 on:** July 09, 2005, 03:27:24 PM »

Wow its a small world!

I just decided to learn to bias amps, and I have a TSL602 as well.

I havent got round to doing the research yet, so heres a good enough place to start, so I second the question!

Skybone · « **Reply #2 on:** July 10, 2005, 06:46:15 AM »

I used to have a TSL122, and wondered about changing valves and biasing etc. I managed to find some info, I think it was from the http://www.marshallampforum.com, but I can't remember.

I copied and pasted this into Word:

Driver = V1 the first preamp tube, farthest from the power tubes.

Phase Inverter = V4 the last preamp tube, closest to the power tubes.

Some people believe in testing preamp tubes for the driver & PI positions, and some don't. If you do, I opt for the matched phase inverter, where they try to match both sides of the triode. I really do not think personally that matched preamp tubs have a noticeable affect on your tone.

AS for biasing the DSL50, Marshall does recommend a bias setting of 45 mA. Here's the Marshall Service Bulletin on biasing the JCM2000 series amps:

Procedure:
1. Make sure the amp is connected to a load with the proper impedance selected.
2. Power up amp on Standby, and let the circuit stabilize for a couple of minutes.
3. Locate the male three pin molex connector (CON2) with the two mini pots (PR1 and PR2) (located on each side of the molex connector) on both ends found at the bottom of tube bay.
4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2.
5. Take amp off Standby with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to mV voltage that is listed below.
6. Repeat steps 4 & 5 for pin3 mV and until pin1 & pin3 mV readings are the same.

Bias Settings:
DSL50 45 mV TSL 601&602 80 mV
DSL100 90 mV DSL 401 1.375V
TSL100/122 90mV DSL201 .675V

Here's a quick bias lesson:
What you want to do is measure the plate voltage (B+) from pin3 of one of the power tubes. It should measure between 450-500V. Now the max plate dissapation (Pa Max) for EL34s is about 25W. To calculate the correct bias, you want to:
(Pa Max/B+)(.7) = 38-45mA depending on the exact plate voltage of your amp. Check and adjust via the molex connector and pots from above.

Hope this helps, good luck![/i]

MDV · « **Reply #3 on:** July 10, 2005, 12:10:17 PM »

Skybone yer a gentleman and a scholar!

Cheers mate.

TwilightOdyssey · « **Reply #4 on:** July 10, 2005, 01:24:01 PM »

You will probably want the amp to warm up for longer than a couple of minutes, too. The tubes will need to stabalize. Usually at work, we let an amp run for about 30-45 minutes before biasing.

PUMPINGDELUXE · « **Reply #5 on:** July 10, 2005, 03:38:56 PM »

Cheers guys !,,,, also ,, what equipment will I need to buy and how much cash will it set me back ?

Skybone · « **Reply #6 on:** July 10, 2005, 07:46:50 PM »

You'll need a multimeter, which costs around about £10-20 from your local Maplins or other electrical stuff retailer, and a set of "watchmakers" screwdrivers, you know, the smallish ones that come in sets of about 6, with 2 "philips" and 4 flats, cost about £5-ish.

Just remember though, there's some pretty HIGH voltages running around in the amp. Be careful.

PUMPINGDELUXE · « **Reply #7 on:** July 11, 2005, 01:52:25 PM »

Cheers guys :D ,,, I didnt really fancy spending £80 on something as simple as a amp re-bais,,,,,
The funny thing is,, when I wanted to sell my amp (I hold my head in shame, how could I even think about it!) they only offered me £100... for a mint condition marshall TSL602,, and they were gunna charge me 80 quid for a re-bais excluding valves :x

:!: !

Eddie Van Halen · « **Reply #8 on:** July 11, 2005, 06:21:18 PM »

Quote from: PUMPINGDELUXE

Cheers guys :D ,,, I didnt really fancy spending £80 on something as simple as a amp re-bais,,,,,
The funny thing is,, when I wanted to sell my amp (I hold my head in shame, how could I even think about it!) they only offered me £100... for a mint condition marshall TSL602,, and they were gunna charge me 80 quid for a re-bais excluding valves :x :!: !

£100! thats daylight robbery!

seriously, they really tried to rip you off there

PUMPINGDELUXE · « **Reply #9 on:** July 11, 2005, 06:23:47 PM »

I Know thats why I just laughed at them and put the phone down

Skybone · « **Reply #10 on:** July 11, 2005, 06:38:23 PM »

I really wish I'd not sold my TSL122, seriously underrated amps IMO.

£100 for a TSL602?????

Laughter was too good for them!

I've seen TSL122's ond 602's go for around £400-£450 2nd hand in shops, taking into consideration a shop has to put VAT & SOME profit (not £300 worth!) so a good price for a part ex would be £300-£350ish, better still, sell it privately. I think I got about £350 px for mine and it was mint.

PUMPINGDELUXE · « **Reply #11 on:** July 11, 2005, 07:09:27 PM »

This sounds a bit stupid,, but im having trouble getting into the amp to bias it..
Do I need to use a chainsaw to get into it !!!!

PUMPINGDELUXE · « **Reply #12 on:** July 12, 2005, 03:03:17 PM »

ok ,, ive cracked this baby open,,,, ,, just trying to bias it now,,,,,, its a bit tricky if youve never done it before

HJM · « **Reply #13 on:** July 16, 2005, 04:02:25 PM »

If you're lazy like me, check out ted weber of weber VSTs tube bbs, there's a bias calculator. Use your ears too!

PUMPINGDELUXE · « **Reply #14 on:** July 17, 2005, 01:06:29 AM »

have you get the web address ?

Author Topic: Biasing a valve amp (Read 17771 times)

TwilightOdyssey