Standalone chupacabra preamp

[fossilshark]

Hi all I am back with more shenanigans. I am finalizing the circuit for this chupacabra rack mount preamp I am building for a friend and I had a few questions.

-In my last thread we basically concluded that an effects loop circuit will be able to output to a power amplifier, How come in the original chupacabra schematic the effects loop is not active? I do not understand why the power amp of this amplifier will basically just accept instrument level signal. This unit is intended to output into a peavey classic series 50/50, should I go with the active effects loop? (see picture)

-Unfortunately the only voltage chart ceriatone provides is the voltage directly on the tubes themselves (see picture) how do I go about choosing the values of the drop resistors and filter caps for each tube stage? I am definitely not using a 550v transformer as I need to fit this in a 1U chassis so it needs to be as small as possible.

Any advice is greatly appreciated.

[fossilshark]

Sorry having technical issues uploading schematics.

[fossilshark]

For whatever reason the forum wont let me upload the .PNG of this schematic and my computer refuses to crop a screenshot of it so I apologize for the link.
https://www.deviantart.com/haftelm/art/Chupacabra-Schematic-CORRECTED-692217353

[kagliostro]

You can show the image you Hosted in some place directly

just copy the link and use it with the "insert image" icon (the Mona Lisa button)




My preferred Image Host is IMGUR (https://imgur.com) (free and they don't delete images after a certain time), there you can host also Large Images with a good resolution that permit to see also little particoulars at the best

Use the green botton on Top at the Left <+ New Post>

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About your question on the output, I'll add the send part of an FXLoop of your choice to avoid problems with long cables and giving more flexibility to the StandAlone preamp

Franco

[fossilshark]

Awesome thank you I will definitely be using an active effects loop.

About the power supply, Ive figured out that the drop resistors I can calculate myself (V1 pin 1 has 228v on it pin 6 has 199v on it ect) I just am wondering how to go about choosing the filter capacitor values for each stage.

Also I am assuming I need a filter choke for the power supply right? how do I choose that value?

[PRR]

...how to go about choosing the filter capacitor values....
...a filter choke for the power supply right? how do I choose that value?

Copy. Find a good plan of similar size/scope and steal Copy values.

[AmberB]

You don't actually need a filter choke, you can use an extra capacitor and resistor off the power supply diodes instead.  With diode rectification, you can use a fairly large first stage filter cap.  For instance, one of the preamp type schematics that I have uses a 100 uf, 450v first stage filter, then a 390 ohm, 2 watt resistor to the second filter stage where the first B+ line comes off.  The second stage filter is a 40uf, 450v cap, and then the next dropping resistor.

[fossilshark]

You don't actually need a filter choke, you can use an extra capacitor and resistor off the power supply diodes instead.  With diode rectification, you can use a fairly large first stage filter cap.  For instance, one of the preamp type schematics that I have uses a 100 uf, 450v first stage filter, then a 390 ohm, 2 watt resistor to the second filter stage where the first B+ line comes off.  The second stage filter is a 40uf, 450v cap, and then the next dropping resistor.

This is a big brain move. I will definitely implement this since iron is expensive these days.

 I have decided to use jcm800 voltage values to calculate the drop resistor values (or yoink them from an existing jcm800 preamp design like PRR suggested) even though this amplifier supposedly has a B+ value of 550vdc (schematic claims they use 500v rated caps on the 550vdc line so I do not know about that chief).

I quickly found out that calculating the voltages using the 550v in the above schematic by hand would be torture so I am just going to pray to the metal gods that I am somewhere in the ballpark of what its supposed to be and it doesnt sound like trash.

[T Wilcox]

My first preamp build a few years back was the Yeti ( pretty much same as Chupa except a couple component values). Used a rack for a router I found in E-waste and gutted it and built circuit in there.
Came out so well I have built >20 rack preamps since then
if you need Time based FX between the pre and power amp use an FX loop that will step down to instrument level for the FX and then step back up to Line level before feeding the power amp. you can design it so it has both
I used this Antek transformer for my Yeti build : https://www.antekinc.com/as-05t240-50va-240v-transformer/

Todd

[kagliostro]

For Stand Alone preamp (also Rack U1) I find that this transformers are interesting

https://www.musikding.de/Toroidal-transformer-230V-270V-14V

Primary 230V, secondary 270V @0,08A and 14V @2A

Franco

[fossilshark]

Wilcox and kaglio I can't believe I forgot toroid trannies existed. That is definitely what I will be going with.

I am almost done designing the power supply and I will draw up a completed schematic for you all to make fun of 

[mwelch55]

I built a 25 watt chupacabra and used the voltages supplied in Ceriatone's voltage spreadsheet.  It really makes a difference when you match the voltages in that spreadsheet.  I liked it so much I built another one.

Mike

[fossilshark]

I built a 25 watt chupacabra and used the voltages supplied in Ceriatone's voltage spreadsheet.  It really makes a difference when you match the voltages in that spreadsheet.  I liked it so much I built another one.

Mike

Yes turns out I am a silly goose. I found the voltage chart and I am designing the power supply to those specs.

[tubenit]

Something to consider for an active FX ............ IF you want to save adding another tube.  Use a mosfet cathode follower.  The CF and tone stack act as a "send" for the active FX.     I've done this on several amps and have been quite happy with this approach.


With respect, Tubenit

[joesatch]

i've put together a few rack preamps lately. i used this PS https://www.ebay.com/itm/255527171842 which rectified and filtered gets me where i need to be around 320VDC (more importantly fits in a 1U chassis) . It has dual filament taps when run in series can drive a 12vdc heater board which also supplies the relays. I have been using the output stage of the soldano X88R on all my builds, requires an additional tube but worth it imo. Preamp is from the BE50 schematic

1U friedman BE preamp finished last week. footswitchable 3 channels Clean, BE, HBE. Also saturation switch (diode clipping).

[fossilshark]

i've put together a few rack preamps lately. i used this PS https://www.ebay.com/itm/255527171842 which rectified and filtered gets me where i need to be around 320VDC (more importantly fits in a 1U chassis) . It has dual filament taps when run in series can drive a 12vdc heater board which also supplies the relays. I have been using the output stage of the soldano X88R on all my builds, requires an additional tube but worth it imo. Preamp is from the BE50 schematic

1U friedman BE preamp finished last week. footswitchable 3 channels Clean, BE, HBE. Also saturation switch (diode clipping).

Yes I have found those but the big issue is I cant find one with a 110V primary. Apparently only europeans need toroidal power transformers

[joesatch]

i've put together a few rack preamps lately. i used this PS https://www.ebay.com/itm/255527171842 which rectified and filtered gets me where i need to be around 320VDC (more importantly fits in a 1U chassis) . It has dual filament taps when run in series can drive a 12vdc heater board which also supplies the relays. I have been using the output stage of the soldano X88R on all my builds, requires an additional tube but worth it imo. Preamp is from the BE50 schematic

1U friedman BE preamp finished last week. footswitchable 3 channels Clean, BE, HBE. Also saturation switch (diode clipping).

Yes I have found those but the big issue is I cant find one with a 110V primary. Apparently only europeans need toroidal power transformers

it is 110V , i live in the US. The 220v 240v is the secondary (which is what you need)

[fossilshark]

I just saw that. I really would like to use that transformer but unfortunately the highest DC voltage I need is 328v for the cathode follower output, so 320v rectified is a bit low.

I am really struggling designing this power supply. I think I am going to draw up what I have and post it for any advice.

[joesatch]

I just saw that. I really would like to use that transformer but unfortunately the highest DC voltage I need is 328v for the cathode follower output, so 320v rectified is a bit low.

I am really struggling designing this power supply. I think I am going to draw up what I have and post it for any advice.

They make a higher one with the same footprint. I've built some SLO's with this one which when rectified will get you about 380vdc.

https://www.ebay.com/itm/373151053911?hash=item56e1893857:g:B-MAAOSw8adcib7g&amdata=enc%3AAQAHAAAAoB7Hvp6%2FIoUI0ry%2BTPyGxKHAvMNnsAVLEjuubLmkQ5FqIlr7nnz%2Fci2ofcrIQB3dTmdKdb9vEuzq7kwvo1xwMOWqJcwkPtLuZPnxMxQomwLHNQupoKdKtfIqCO6Ym7BYP5u%2BAJZXs04HGzhbtNv%2FwEZpwJMTdhQz%2F78m9cEzeWdG7vwDYjFqCqhwnFGNuDJASI%2BOAyDQg8LwwUqWtx%2B5YjA%3D%7Ctkp%3ABk9SR9TPw73GYQ

I put the lead and clean channel from an X88R into a 1U rack using that PT

[fossilshark]

They make a higher one with the same footprint. I've built some SLO's with this one which when rectified will get you about 380vdc.

Oh man this is perfect. So if I use this I can use 380v off the rectifier to calculate my voltage drop resistors?

[joesatch]

Ya fully filtered depending on how many preamp tubes you'd probably get 370vdc B+ then use the dropping resistors you need. The even make one that goes higher,  the AS-05T320 for 320V secondary will get you over 400vdc rectified

[fossilshark]

perfect thank you so much. I will finalize the schematic and post it here.