custom 100w 4 channel amp with bassman, twin reverb, delux and champ preamps

[jasonttaylor]

hi i want to scratch build a 100w amp with 4 channels, each channel with an iconic fender tube preamps. i plan to use the oroginal preamps circuits from the bassman 100, the fender twin reverb, the fender deluxe and im undecided apon a 4th preamp yet. i plan to use the tube preamps and power them with a solid state power amp. i have never worked with tubes before of built any substantial amps altho i know enough theory behind them. i am awear i might stumble accross a gain defference issue from each preamp and i was wondering if you could use op amps to buffer the outputs of each preamps to make the output signal from the preamps easyer to work with in the solid state amps by getting all the preamps to deliver similar output power levels. on compairing the peramps of the 3 preamps i have chosen so for i have noticed that they are all verry similar and i wonder if the tone or sound i want from each channel will be affected greatly by the lack of a tube power amp or if i can expect them to still sound similar to the amps emulated my each channel?

[nickb]

HI Jason and welcome to the forum.

That's a LOT of preamps It might be a good idea to start with just one and build up from there.

If you have all four channels how do you plan to configure the inputs as they could be driving more than one channel? Paralleling things up will affect the tone unless you take steps to mitigate it. Are these going to be switched or all running in parallel? Potentially, that's lot of front panel controls. With careful thought you may be able to use switching to vary between the topologies and save on real estate and cost.

Have you taken a look at the schematics yet? There is not much difference between the Deluxe Reverb and the Twin Reverb so you might want to choose another. Also both have four inputs - you might not want that many.

There will be gain differences but if you can put an appropriate gain correction on each output as necessary without too much difficulty. Since tubes can easily output tens of volts that will instantly destroy opamps you will need to think about the signal levels and take steps to protect the SS devices.

What do you plan to use for the power amp?

[olddawg]

You might want to look around for a Fender PA 135 or similar and mod it. They are basically a Showman/Twin Reverb with 4 input channels. They even look like one. I see the around a lot for $150 or so. It would be a great starting point and everything is there already including a tube driven reverb.

[Justin Thomas]

So, I have to ask: which channel on the Twin Reverb: the normal or vibrato? One has two stages, the other has three. Also, the Bassman 100 preamp is the same as the Twin' s normal, for all intents and purposes. I'd put in the preamp from a 50W Bassman head, AB165 or later. I did that to MY Bassman 100...

Also: which Deluxe? 5E3, Brown? And if Blackface, same question as for the Twin - Normal or Vibrato?
And, Champ: pentode? Later Tweed, or BF?

These options will determine the number of actual tubes you need... also, you noted the similarities of the persons in these. I bet a lot of the magic happens later in the amp. If I may share my own personal opinions, I'd have a Pentode Champ channel, a 5E3 channel, the AB165 Normal channel, and the Twin' s Reverb channel. Which would require... 4 12AX7s & a 6SJ7. With a 100W power section, that'd be killer! As far as combining, I think the Bassman & Twin' s could be numbered; maybe the Champ as well. The Deluxe I don't know about; try it and see. You'd still need 8 inputs to do that, though...

Ambitious first project; good advice on the PA135, though I'd hit a PA100 if I could. Price is about the same. Use one of the 12AX7 slots for an EF86 instead of the 6SJ7 and you still have a killer channel.

Good luck!

Justin

[jasonttaylor]

i have looked at schinatics and i got the idea from the fender bassman AB165 that has the seperate bass and guitar channels and i thaught why not switch out the bass preamp with another fender type of preamp like the twin reverb. i had planned on using a TDA7498 as a power amp, i know its suposed to be a dual 100w amp but id never take up to that point. i plan to use one of the output channels and only use the second if i want to record a stereo track by using two different cabs. about the output of the tube preamps. could one not use a high resistor value before the op amp or use a risistor to ground to drop the output level to a usable level to make op amp buffereing possible? Do you think scaling down would be a better option, have two different preamps instead of 4?

[jasonttaylor]

thanx for the insite. i dont have axess to alot of fender amps so im bassing my amp choices off of youtube videos of amps. so thanx for you insite. ill look at the schimatic for each one

[nickb]

i ... could one not use a high resistor value before the op amp or use a risistor to ground to drop the output level to a usable level to make op amp buffereing possible? Do you think scaling down would be a better option, have two different preamps instead of 4?

That's one way to do it but really you are designing ahead of yourself. You need to know thesignal levels expected and required and also the impedances on the the source (preamp) side and the power amp input side to design an appropriate scheme.

Start by getting your preamp section drawn then you can answer the first part. Then get the datasheet for the power amp board and find out the other half of the information. The datasheet should also specify the absolute maximum input level and you need to be sure that level cannot ever be reached. One thing to note is that class D amps do not like to be driven to clipping so make sure you don't let that happen either.