Robyn T-240D plate mod ? for Justin

[Shadetree Mechanic]

Just got a Robyn T-240D and the plate modulation conversion sounds interesting. Does anyone have any information on this?

 

[Handy Andy]

ARE YOU SURE YOU KNOW WHAT YOU ARE DOING!?

This is an AC HV version of the RWOB mod others used...but for Bipolars...

Just got a Robyn T-240D and the plate modulation conversion sounds interesting. Does anyone have any information on this?

Enclosed in the Final TX strip of that radio so you can see how the bypass works...PLEASE BE CAREFUL and use HV coax cable - it can poke you and destroy tubes...



I wish you the best of luck...

 

[Shadetree Mechanic]

ARE YOU SURE YOU KNOW WHAT YOU ARE DOING!?

This is an AC HV version of the RWOB mod others used...but for Bipolars...



View attachment 25306
Enclosed in the Final TX strip of that radio so you can see how the bypass works...PLEASE BE CAREFUL and use HV coax cable - it can poke you and destroy tubes...

View attachment 25308

I wish you the best of luck...

I know just enough to be dangerous. lol mainly I am just curious what it does and why it works. Would mini8 be HV or what would be needed? This doesn't look hard to do, what adjustments would be needed after doing this mod?

 

[Handy Andy]

It isn't the best mod for it.

There are no real adjustments after this mod, for all the Modulator does, this mod bypass'es the chokehold they placed in it - and you have to ride the Mic gain to keep the thing from getting too excessive, in both modulation and plate dissipation...

Referring to Grid and Plate issues.

They "choke the mod" down, lower to allow the tube - both Modulator and the Final - to dissipate power for their size inside that box - else you'd need a larger tube and the associated power supply issues...

Note the schematic and the mod itself - they use diodes to keep it "one way" towards the Final tube.

(Man I'm not a tube guy so I can't believe I'm saying this...)​

When you broadband it, you're adding more audio -and adding an FM component similar to clipping on a Bipolar, only you're heating up the Final Tube and introducing another path for parasitic problems and one day it may just go SNAP and the tube is toast.

In the Transistor logic view, you're placing Modulation on both the Base and Collector at the same time so what you do is add a compression effect - by Phase modulating the output of the Driver, into the Final - and the Final Tube itself - you hear unfiltered audio resonating with the Carrier the Driver sends to it, along with the Grids resonating with and offsetting, Plate modulation - so you're generating / making the all the grids in the tube work

There's a neon light as modulation peak indicator - if it's on too long or stays on all the time, you have a squeal or oscillation you'll need to track down. Some people back in the day called it their on-air light - I'll let you figure that one out...

The diodes' if they blow, may not blow open - they may blow shorted - think about the sharing of current...

It's just a lot more work on that last stage tube...

 

[Shadetree Mechanic]

It isn't the best mod for it.

There are no real adjustments after this mod, for all the Modulator does, this mod bypass'es the chokehold they placed in it - and you have to ride the Mic gain to keep the thing from getting too excessive, in both modulation and plate dissipation...

Referring to Grid and Plate issues.

They "choke the mod" down, lower to allow the tube - both Modulator and the Final - to dissipate power for their size inside that box - else you'd need a larger tube and the associated power supply issues...

Note the schematic and the mod itself - they use diodes to keep it "one way" towards the Final tube.

(Man I'm not a tube guy so I can't believe I'm saying this...)​

When you broadband it, you're adding more audio -and adding an FM component similar to clipping on a Bipolar, only you're heating up the Final Tube and introducing another path for parasitic problems and one day it may just go SNAP and the tube is toast.

In the Transistor logic view, you're placing Modulation on both the Base and Collector at the same time so what you do is add a compression effect - by Phase modulating the output of the Driver, into the Final - and the Final Tube itself - you hear unfiltered audio resonating with the Carrier the Driver sends to it, along with the Grids resonating with and offsetting, Plate modulation - so you're generating / making the all the grids in the tube work

There's a neon light as modulation peak indicator - if it's on too long or stays on all the time, you have a squeal or oscillation you'll need to track down. Some people back in the day called it their on-air light - I'll let you figure that one out...

The diodes' if they blow, may not blow open - they may blow shorted - think about the sharing of current...

It's just a lot more work on that last stage tube...

Thanks for the explanation, I was able to turn up VR4 modulation and the mic amp. The MOD light still flickers, its just on more than it is off. Before it was off more than it was on.

 

[Handy Andy]

You just got it, don't blow it up before you get a chance to use it for real...

The mod simply adds modulation to more of the tubes grids - not a big deal - but you're kinda playing with fire if you don't think thru the mod. You'd have better luck with as you discovered - the Mod limiter - and then just leave it at that - you don't have a lot of headroom in the Robyn for any big power swings and "volting" up finals - you're at 100+ volts now, and you have to think of the wattage at that voltage - it's a lot of heat to get rid of, inside that tube...

 

[BJ radionut]

Chris/STM: I am with Andy here that rig is screen modulated...yea the Plate mod could make a difference, but you have to add the extra stress you will add to the tubes when doing so.
When you could buy those 6BQ5A's for $.25 each for NOS RCA's or Sylvania's big deal! No longer the case, those and the EL84/6BQ5A (Audio Amps guys) are really in demand and have driven the pricing way up, even the Russian substitutes have risen much in the last few years.(last production dated's are in the mid 60's for RCA/GE/SYL's)
So toss up! I would shop around at a good tube dealer(not some random Ebay'er)
Tube Depot/ Antique radio supply/K5SVC...Electro Harmonics...etc. and buy a couple new ones, plug those in and see what see does. You want the 6BQ5(A) that is the Audio rated tube(IMHO) but good brand GE/ RCA's or Syl's 6BQ5's are good also.
All the Best
Gary

 

[BJ radionut]

Couple other Notes: Best tubes(IMHO) for usage in the older "Tuber's...No specific order:
RCA
GE
Sylvania
Amprex (Holland made versions) Excellent but hard to find nowadays also.
Philips/ECG...these are actually Sylvania's...When Sylvania got out of tube biz in late 70's/80's these production facilities were purchased and name re-branded.
Svetlana/Winged "C"...Russian made substitutes...just make sure they are DIRECT sub's...That's where good dealer is important and know's the correct direct replacements.

Some New Manufactured/Current production versions...
Electro Harmonics/Mullard/Tesla...
I know the Audio guys pay big bucks for some of these, but they are wanting TRUE Matched pairs/triplets...etc...for tuber mono/stereo/Guitar amps etc... where that is critical to rich sound quality.
Tungsol ( I think is another re-branded) Some US/some Japan-China...the're are new and old versions around.
So just some more info...for what it's worth
All the Best
Gary

 

[Handy Andy]

That's just awesome - you're my go-to guy for ideas and concepts on re-tubing.

Thanks! That's some good stuff to know. It doesn't bring my ex-XYL back (GAWD maybe? Hope not) - but it tells me there were a lot of fans of tubes - back then - that are still around now.

 

[JüstînSîxx]

Chris/STM: I am with Andy here that rig is screen modulated...yea the Plate mod could make a difference, but you have to add the extra stress you will add to the tubes when doing so.
When you could buy those 6BQ5A's for $.25 each for NOS RCA's or Sylvania's big deal! No longer the case, those and the EL84/6BQ5A (Audio Amps guys) are really in demand and have driven the pricing way up, even the Russian substitutes have risen much in the last few years.(last production dated's are in the mid 60's for RCA/GE/SYL's)
So toss up! I would shop around at a good tube dealer(not some random Ebay'er)
Tube Depot/ Antique radio supply/K5SVC...Electro Harmonics...etc. and buy a couple new ones, plug those in and see what see does. You want the 6BQ5(A) that is the Audio rated tube(IMHO) but good brand GE/ RCA's or Syl's 6BQ5's are good also.
All the Best
Gary

New Justin here lol, i was wondering if you could maybe post the full schematic to this unit for me please? Been scouring google all night trying to fid one lol. Basically i was given and saved one of these Robyn240s from the landfill bout a month ago. Im a guitarist, and a music junkie(i live for it lol!) but this thing has a few blown caps inside, pretty sure it was rained on a few times, i see some components caps/resistors i assume have been snipped at some point, oh and it was LOADED to the hilt with friggin stink bugs. So i decided im gonna salvage what components I could use, and idk what Ill do with whats left. This has became a new hobby, ive built a couple simple amplifiers recently. But i digress, i want to take the Transformer, and the tubes out, and im gonna repurpose them into a pre-amp to make my stereo and my main guitar Amplifiers Hybrid Tube/Solid State. So i figured id ask you guys because so far other then a tuning manual that had a piece if schematic, this is the closest ive came to a full schematic lol! Im not new to forums, so i apologize in advance for dredging up an old thread. Buut maybe itll help someone else in the future, and save room on your forum! Thank you in advance if you still have the schematic, either way cheers!

 

[JüstînSîxx]

Heres some photos of the unit. Shes definitely saw better days thats for sure. I thought about recapping/tubing and any other repairs it would need at first. Then i found out it came equipped with good tubes i could use for my preamp build, and saw some other vimtage goodies as far as components go, and decided to upcycle it lol!

 

[Dmans]

http://www.cbradioinfo.com/cbtricksoldsite/radios/robyn/t_240d/graphics/robyn_t_240d_om_sch.pdf

73
David

 

[danmcclain]

I have one of these t240's and the receive is out. Where do I begin? It's a friggin spaghetti bowl down there!!!

Test the tubes...

 

[nomadradio]

I am with Andy here that rig is screen modulated...

Screeeech! (Needle ripping across an LP)

This radio uses a "Heising" modulator. The primary winding of the speaker transformer serves as an audio-frequency choke. The positive and negative excursions of the 6BQ5 anode are what power the final tube, connected directly to the 'cold' end of the final's plate choke. The schematic is pretty clear about this.

Technically it's "high-level" modulation already since it's applied to the class-C final's anode supply. But it never performs as well as a real modulation transformer. One that has a primary on the 6BQ5 plate, and a secondary winding in series with the final tube's B+ supply.

This is what's found in a Browning or Tram. Does make that transformer more expensive.

Another radio where you'll find the "Heising" modulation circuit is the DAK Mark 9.

Feel free to wiki it, sometimes also called "constant-current" modulation.

73

 

[BJ radionut]

Chris: Thanks for the correction/clarification...have to look up...
All the Best
Gary

Screeeech! (Needle ripping across an LP)

Sorry!