Why are the tubes in all amp layouts in the back of the amp?

[Gamedojo]

Hey there. First post, long time "lurker"...

I'm designing a layout for an amp, and want to place the preamp tubes between the board and the controls. Is there anything I might be missing here? I want really short leads, which is the reason for the tube placement, and am concerned that there might be a issue with possible crosstalk or something since the tubes are so close to the input.

Any issues? or are tubes placed in back in most amps because it makes them easier to change and be cooler (temp)?

Tyler

[Johnhenry]

I have seen a few amp's with preamp tube's up front on the chassis, but i think you hit the nail on the head with, they run cooler and easier to service in the back ! and you might have some crosstalk with the high voltage being close to your EQ !
Just my 2Cents, some one else may answer this better !
Johnhenry

[Bob-I]

Soldano puts the preamp tubes in front. The key is that the front is open, just wire mesh covering the tube, plus they're very careful on the lead dress in front.

Here's a SLOClone I built some time ago. Notice how busy the front of the amp is. The bundle of wires in the middle, with the black tie wrap, is the tone stack wiring. It was a tad difficult to keep these wires away from the signal wires to the tubes, but the amp sounds great and is VERY stable.

[Andy Le Blanc]

used to be that amps were the other way round
everything was in the back...
it was a new industry standard to move the controls to the front
and the tubes to the back .....the changes in the fender models
are a good example.... true point to point without component boards
had its own demands..... putting the parts on a board in the amp
made it possible to optimize a layout and consumer features..
like knobs were someone could use them.... and good tube ventilation

[Richie]

sunn amps put tubes up front for preamp,and PI in the middle and power amp section on the back.
Sound City amps have the preamp moved closer to the middle,with the power tubes along the back.. not easy to change the preamp tubes. And the power tubes have a shield.

[JammyDodger]

To All,

This is an interesting project for me. I am going to be building a fairly simple 2xEL84 amp with a tube recifier based on a Hammond amp. I am thinking of arranging the tubes in a 'U' shape around the pre-amp board to keep my low-level signal chain as short as possible.
The transformers will be on either side of the chassis for weight balance. I am trying for a very compact amp combo, as small as possible using a single 12" speaker.

Any thoughts?

Cheers, Mike

[Alexo]

I know it sure looks like a good idea to put the transformers on either side of the chassis for balance, but then you end up with the preamp circuitry near the power supply or near the OT leads, neither of which is ideal.

Having said that, it has been done effectively.

[ampdoc1]

I made a Dumble/Trainwreck hybrid into an old Fender Bandmaster chassis and mounted the preamp tubes right behind the controls. I put the PI and also the tube for a "dumbleator' circuit at the back next to the power tubes. Didn't have any noise or hum problems at all.

ampdoc

[briane]

Having said that, it has been done effectively.

Yes, it has been done, look at the old silvertone amps.

In any case every time I did that it squealed like a stuck pig. I dont do that anymore. I keep my OT over the PI, or even closer to the PS if possible.

[Andy Le Blanc]

Ive disassembled a couple army surplus amps
they were compact..... made a hiwatt look clumsy
critical wire runs were put through braided wire tubes
the same stuff you see as solder wick
even the grounding contacts were painted as inspected
another solution I've seen inside McIntosh amps
the componant board is vertical
resistors and ceramic caps on one side
large poly caps on the other side with interconnects running
under the caps at right angles
all othe interconnects are on the chassis running at right angles to those
both use fully potted trannies

[sliberty]

I've built several amps with the tubes (all of the tubes) in the front. In a traditional head cabinet, this is not very convenient as the trannies are in the way of reaching the tubes from the back of the cab. But if you are using a cage, it is just fine since you have to take the cage off to service in any case. From a layout point of view, it can result in a very nice layout actually.