Input Stage Advice?

[Lonzo]

Hello All

While I'm probably getting in over my head....I have a Laney Pro Tube 30 AOR Series II head and am seeking some advice on the first input stage (high input).

What I "think" I've found is that the high input only uses half of V1, with the input signal hitting a 68K input resistor to V1 on the B side. The low input completely bypasses V1 and goes directly to V2 (via a 68K input resistor).

I would like to get a little hotter signal to the preamp without adding a boost pedal on the front end. Can both halves of V1 be used to accomplish this? Would it be better to reduce the value of the 68K input resistor? Or...am I completely out of the ballpark?

Thanks for reading. Any help is greatly appreciated.

-lonzo

[kg]

is the 1st half of v1 not used for anything else?

[Don Moose]

They used to 'hot rod' marshalls by jumpering the channels in exactly the same way.

I'm working without a schematic here, so I'm gonna have to be vague.

I would try to go:

High_Input->normal V1 circuit->new coupling cap->new volume pot->duplicate of V1 circuit using spare triode->old V1 coupling cap, using shielded cable for most of the new wiring.

What's your experience/comfort level working with the definitely lethal voltages inside a tube amp?

[Lonzo]

First Stage Help....

KG - It appears the other half of the first 12AX7 is not in use - maybe parallel to the next stage? It has wires from the other pins (plates I think) that are soldered to the second preamp tube.

Don Moose - I find it ironic that I just moved from Avon, IN to KS and get a post from a fellow Hoosier. Ever visit 6 Strings Down in Plainfield? IMO it is the best shop in the midwest....Back to the business at hand. In regard to your question, I am a tweeker that understands what can kill me if I touch it. My chopstick touches much more than my hands do... My experience is primarily with component swaps but I have a good understanding of how things work.

It seems odd that the schemo show the 68K input resisitor entering at V1B. Isn't V1 the second half of the tube? I find the 68K soldered to pin 2 of V1....am I missing something? Could I have the wrong schemo? It's labeled correctly (at least the same as what my amp is labeled). So.....

I've tried some of the "classic" Marshall 2204 mods to no avail. I placed a 330pf cap across the 68K input resistor and didn't like the affect on the lows / mids. I'm experimenting with the 10K Cathode resistor in the first gain stage as well. I currently have it down to 4.7K and am trying to dial in something....

I'm beginning to think I should just hit the front end a little harder - with a pedal. I don't mid doing that, I just thought the unused portion of V1 could be used for something, useful.

Again, thanks for reading and many thanks for your advice.

-lonzo

[Leka]

Hi, check this thread:

http://music-electronics-forum.com/showthread.php?t=54

[Don Moose]

Depending on how the schematic draftsman decided to divide things, the tubes could be numbered such that the triodes in 2 12Ax7 types are V1, V2, V3, V4 or V1A, V1B, V2A, V2B.

The norm is V1A, V1B. The two triodes are symmetrical, so most people make V?A the pin 1,2,3 triode and V?B the 6,7,8 one. It doesn't matter, though.

If I had a schemo, it would help - working blind is a PITA.

-- I can't claim Hoosierness, I've only been here about 2 years. 14 States in 12 years - I don't recommend it.

[Leka]

If I had a schemo, it would help - working blind is a PITA.

Now you have it.

[d95err]

Both triodes of V1 are definately in use. The Hi input uses both triodes in series. So, the amp already gets as much gain as is practical from V1.

[Lonzo]

Thanks for the help!

Leka - thanks for posting the schematic.

Thanks to all for your time and advice.

I still don't see where V1 is in full use. Anyone care to provide a quick tutorial?

Thanks,
Lonzo

[d95err]

V1 is simply two cascaded gain stages. The signal goes from the High input to V1a. Then through the coupling cap (C3) to the "Preamp Gain 1" pot and into V1b. From V1b, the signal goes through another coupling cap (C4), through the Low input and to the "Preamp Gain 2" pot.

When nothing is connected to the Low input jack, the signal from V1b passes through it. When a plug is inserted into the Low input jack, the signal from V1b is broken and the signal goes directly to "Preamp Gain 2".

[Leka]

Leka - thanks for posting the schematic.

Thanks to all for your time and advice.

I still don't see where V1 is in full use. Anyone care to provide a quick tutorial?

Thanks,
Lonzo

If you go through the link I posted earlier in this thread, you'll find out that the AOR 100 is quite the same amp with just bigger power amp.
There is very good info on how to get better sound from this Laney.
I made the suggested mods and now my amp rocks!

Cheers,
- Leka

[Don Moose]

(thanks Leka)

As noted, all the triodes are busy, so nothing there.

After you've been through the thread Leka's pointing to, you might think about:

- reducing the value of R5 (tack a resistor in parallel - 270k working down to 100k)
- increasing the value of that first 100K log pot - maybe a 250k, up to 1M?
- increasing the value of R4 and R10 - R10 first; don't recommend going over about 150k as it gets noisy, but I've seen up to 220k plate resistors.

Each of these steps will either reduce interstage attenuation (steps 1 and 2) or actually turn up the gain (step 3). Since gain amplifies noise as much as it does signal, try that last.

Seriously, though, Leka's thread first.

OK, about doing the mods - this is a PCB amp and traces are fragile. When possible, leave the original component's lead in place and tack the new component to the stub - at least until you're happy with the result.

Hope this helps!

[Lonzo]

Holy Cow!

Many thanks to everyone for the time and assistance. I read through the thread a couple of times before I bought the amp and thought there was some relevance there. I'll disect it more closely by your recommendations.

Don Moose - thanks for the Hoosier hospitality - even if you are a transplant or just passing through. I've pulled a few boards and appreciate the delicate touch that must be used to safely swap components out. Thanks for the advice on the mods and the process. You've been a great help.

d95err - thanks for the explanation on cascaded gain stages. I always thought it would be more complicated (at least more complicated looking).

Leka - thanks again for posting the schemo and your suggestions.

I'll post my results once I've tried a few of your suggestions.

cheers,
'zo

[Lonzo]

Wrong Schematic

Hi everyone

I looked at the schematic more closeley and found that the one posted was for the AOR Series II. My amp is the Pro Tube Series II - PT30MV.

Here's a link to the schematic:

http://www.schematicheaven.com/newam...ube30_PT30.pdf

The schematic that was posted for the AOR indeed shows the use of both halves of V1. The PT30MV schematic does show only half of V1 in use, right?

Sorry for the confusion, I should have posted this link in the first place....but I just found it again.

Thanks,
Lonzo

[kg]

The schematic that was posted for the AOR indeed shows the use of both halves of V1. The PT30MV schematic does show only half of V1 in use, right?

it appears that this is true: i do not see v1a in use on the schemo.

of course verification via following the pins to the traces would be proof.

if it ends up being the case you could strap it up as a cascaded gain stage fairly easily.

ken