I'm looking for half-power switch, and HRM/Non-HRM

[Bob Simpson]

switching capabilities for a 4X6L6 BM 88 HRM.

Are there schematics or layout drawings available for these circuit mods?

Bob

[Structo]

Bob, how about pulling a pair of tubes for half power?

[Bob Simpson]

Yes, I know that will work, one from each side, and switch the output impedance selector ( either up one, or down one, I never can remember which way... ), but not optimum. Then I've got a pair of tubes rolling around on their own...

I think one of the ceriatone d-types has this feature...

Routing around the HRM board would also be a good thing to have, I'm thinkin'...

After all, HRM's are harder to play, right? Like I need that...

Bob

[ampdork]

https://tubeamparchive.com/viewtopic.ph ... highlight=

I already posted this several months ago...

It works and AFAIK there is no other way to achieve this.

There is a lot more to each amp other than the HRM but the rst is rather easy compared to the above.

If you need help with it email me.

Good luck.

[briane]

Hey bob,

I am also interested in how this was done in the originals.

From what I can gleem its basically through a DPDT switch, so I can only assume 1/2 the switch for each tube...So common sense tells me a tube was shunted by interrupting just one wire. Personally I dont know which wire that is, I have been somewhat assuming it was the cathode ground on the power tube....though I suppose it could be the pin feeding the OT...

I'lll take a look at the ctones...maybe nik has a drawing out there.

Any help would be appreciated.

[Bob-I]

IMHO turning off 2 of the power tubes does almost nothing, can't even hear the drop in volume, just a little less girth. A better mod is to switch from pentode to triode mode by connecting the plates to the screens.

JMO

[talbany]

Pretty simple DPDT switch lifts ground on the cathodes on 2 tubes Pin 8 on 6L's so they don't conduct...It doesn't have to be Heavy duty since not much current..If you want to have the output section break sooner and be a bit smoother you can do fixed bias/cathode bias switch which for me I prefuer this over the triode method (Just me).. Bob-I is right not much difference in volume like 3db at low volumes...You can do something like this where you have a pair of 6L's and 6V's /El 34's whatever.. switchable between the 2 as well as cathode bias /fixed bias either way....It's for a 50 but thought I'd post anyway...


Tony VVT

[benoit]

I know it's not often discussed in the Dumble forum but what about VVR? I can't remember now if he said it could handle up to 50 watters or 100, but he might have some ideas. That would be sweet because you'd be able to dial it right where you wanted it volume wise after dialing in the tone you want. I have no personal experience with it but all reports I've seen say it's probably the most transparent attenuation option available.

[Bob-I]

I know it's not often discussed in the Dumble forum but what about VVR? I can't remember now if he said it could handle up to 50 watters or 100, but he might have some ideas. That would be sweet because you'd be able to dial it right where you wanted it volume wise after dialing in the tone you want. I have no personal experience with it but all reports I've seen say it's probably the most transparent attenuation option available.

Honestly I find no need for any of this. I can run my master down to about #2 and still get great tone. If I want to play even lower volumes a volume pedal in the loop does the trick. Bring the master up to 10:00 and back off the volume pedal to whisper levels.

[dogears]

Good post Shad.

For the record, Glaswerks has been completely and effectively switching every facet of non HRM to HRM for years. Don't forget that the clean master is seperate and the bright cap (which always is better there IMO) needs to be designed so it is not a bleed to ground when non HRM is on.

Also, seperate pre-od networks with seperate trimmers.

V2B coupling size changes, as does the impedence it drives (250K vs 100K pot). You can short things in series to get approximate values. You can use a 250Ka pot, like the HRM, but you can jumper around the 100K feeding it. Better yet, try a 100K pot, scraped up to 160K or so. Then use two resistors in series to get the impedences correct. You need series caps as well. Short one to get the bigger non HRM value. Use a larger one in HRM if you need work with lower impedence than the 350K standard. Best solution is a custom tweaked dual gang pot with bioth 100KB and 250KA elements.

Lastly no snubbers always sounds better on HRM to me.

So, you need several relays to make it work best.

Your schematic is good though. No fault there!

You said it was not old hat, but for some it is. Now that Gary is doing the SODII where he has a dedicated non HRM and HRM preamp tube and circuitry, I can post this. THe new solution, seperate discrete circuits with no shared controls, is the superlative way IMO. Nobody waited to reverse engineer anything. The original switching design was the result of at least 12 months of collaboration between Gary and I. We did not feel like sharing so that certain foreign schematic trollers would pinch it.

You could make a new board and use this scheme to switch out the hrm (or in).
Put a relay on the board and associated parts shown here. Use a 100k trim pot for the "ratio" for the non hrm side.

Of course there are numerous other things you can incorporate (and should imo) here and this shows nothing more then the hrm stack being used or bypassed. I am aware of all the rest that this does not address in regards to the difference between the circuits.

This basic idea does work just fine for the hrm switching but of course I am sure the people around here could find fault in it or do it better.....after all I am just another AMPDORK! ;O)
The other stuff could probably be easily figured out....well in fact I know it can. This one is a bit trickier then most of the rest and it is simple and obvious.....So the rest should be able to be hashed out in discussion to get both modes singing nicely.
I am in fact astounded this is not old hat around here...Maybe I ought to have said something sooner?

If one goes deeper another hint is that when switching the V1 nfb put the cap one the plate before the relay....that should be obvious but ...
Keep on in this direction of thought and the results can be well worth it!!

OR....you can do it the other way....that is wait around for dumble to do it first and reverse it. ;O)

Hope it helps. Mileage may vary. I accept no responsibility for the kludging of anyones amp. Use at own risk (cause it can be too cool) and ENJOY!

http://img25.imageshack.us/img25/8533/hrmtrick.gif

https://tubeamparchive.com/viewtopic.ph ... highlight=

I already posted this several months ago...

It works and AFAIK there is no other way to achieve this.

There is a lot more to each amp other than the HRM but the rst is rather easy compared to the above.

If you need help with it email me.

Good luck.

[benoit]

I know it's not often discussed in the Dumble forum but what about VVR? I can't remember now if he said it could handle up to 50 watters or 100, but he might have some ideas. That would be sweet because you'd be able to dial it right where you wanted it volume wise after dialing in the tone you want. I have no personal experience with it but all reports I've seen say it's probably the most transparent attenuation option available.

Honestly I find no need for any of this. I can run my master down to about #2 and still get great tone. If I want to play even lower volumes a volume pedal in the loop does the trick. Bring the master up to 10:00 and back off the volume pedal to whisper levels.

Bob, I have a 100W HRM and fully agree with you. I just figured if the guy REALLY wanted a low power option that might make him happy. But I should have said what you did, which is that the amp should still be smokin' low on the master.