SRPP line amp

[Kamel]

srpp line amp based on the jakob design, with jakob authorization, but modified to use different triode easily.
adding a tap at around 75v for the heater to manage the Vhk specification.
can use 12,6v or 6,3v heater.
use input step up transformer (1:2 xs1100 or 5402 reversed) to balance the step down output.
i would like to use a 500k (maybe lower? )pot after the input treansformer to have the ability of instrument (mainly synthesizer) or line input, and use a switched jack after the transformer to enter instrument (guitar or bass).
lastly, i will use a switch to change R5 to optimze the output load of the SRPP.

comments are welcome

 

[Kamel]

here is the layout

 

[EEMO1]

yeah!

I've been thinking about building one, 2 channels to use on the master bus...

 

[Kamel]

i try to make it versatil, but it's not so easy, due to the impedance range of the possible input, i would like to know what is the output Z of an electric passive bass/guitar, i know that there is not single answer, but, wich input Z will not hurt too much the output Z of a guitar?

 

[alexc]

For a passive guitar/bass pickup,  100Kohms or so  minimum input impedance for direct connection.

I have 47Kohm specified input impedance 'DI'  (transformer) and they work but are not great.
Work OK for keyboards and so on.

 

[Sorr]

For a passive instrument input, 1 Meg or more.
You dont need C1
The C3 across C2 trick is a pain to get right.

 

[alexc]

How do you achieve a 1Mohm or higher input with a transformer coupled to the tube grid?

 

[gyraf]

Bypass transformer for instrument input - it's not balanced anyway..

Look at G9 input circuit for inspiration

Jakob E.

 

[alexc]

Yep.  We all love the G9 

Thanks Jakob

 

[Kamel]

yes we all love the G9

sorry to reply so later, but yes, the Tx is bypassed by a switched jack ala G9 input, trick used on many DIY, even commercial, tube pre.
I think the input cap is usefull, if you are using an active guitar/bass and there is a small DC offset

 

[pyjaman]

Hello,

I think the input cap is usefull, if you are using an active guitar/bass and there is a small DC offset

Correct me if I'm wrong but I think tubes won't care about small DC offset ( if small enough, that could only change tube bias in a negligible way ) proof for it: active instruments can actually be plugged directly in tubes guitare amps (usualy without coupling cap) without any problem.
Laurent.