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10/29/2004 12:19 PM
Ferdinand The Bull
I've been playing my recently built monster. It's based on the early 50s tweed Pro (5C5), with 6550 power tubes. Holy COW it's good.  
I used an old hi-fi OT with 8ohm, 16ohm, and 70V taps. I'm using the 16 ohm with two P12Q drivers in series.  
It suddenly occured to me that I may be in danger of toasting the drivers (rated @ 40W?) if I run them in series. I assume this amp puts out at least 40W.  
I guess my questions are about the OT primary impedance of this beast.  
1. What's a ballpark prim. imp. for this application?  
2. Can anyone take an educated guess at what the prim. imp. of my hi-fi OT is? (my guess was 6.6k for P/P 6550)  
3. 8ohm tap with 4ohm load (parallel drivers) and be done with it?
10/29/2004 11:37 PM
Dr. Photon

the power handling should be OK. both speakers share the power equally (since eah has the same current (the full output) going through it, but only half of the voltage), so they should handle 80 watts total (and these are guitar speakers....everything sounds good when pushed a 'lil)  
The optimum primary impedance of the 6550's depends on both the plate and screen currents. the primary impedance is not very critical, it'll work fine as long as it's correct within an order of magnitude.  
You can measure the impednace ratio of the transformer. Simply feed it an AC signal (from a function generator or stepped down AC) to the primary, and measure the voltage on one of the seondaries. The ratio of these two voltages is the turns ratio. square this and you get the ratio of the impedances. the "impedance" of a transformer is a ratio. if you have a 6.6K p-p transformer with an 8 ohm tap, and you plug in a 4 ohm load, it's presenting the tubes with a 3.3K load  
If it's sounding awsome and nothing's getting really hot/burning/melting/red plating then I'd just leave it alone. Mismathing the load shouldn't make it blow it
10/30/2004 12:46 AM

Hi guys!  
Just to make Dr Photon's explanation a tad more detailed.  
Ferdinand the bull, well done on your amp, i like 6550, what brand are you running?  
Best regards.  
10/30/2004 7:07 AM

I'm also interested in this tread.  
I made a SLO 50w clone, wich at first had a pair of sovtek 5881wxt (KRAP). For the 6L6GC/5881 plate load, my OT has 4K4 ohms. B+= 480v  
Recently I retubed my monster with Tung Sol 6550.  
So, I loaded the 6550 plates with my 16 ohm cab + the 8 ohms OT tap, resoulting Load= 8K8.  
My questions:  
What's the recomended "center point" for a pair of 6550?  
What would be better/closer, 8K8 or 4K4?  
Would be safe/soud wise to try 4K4?  
Thanxs. SERGIO  
BTW, TUNG SOL 6550 makes my sheffields kick like a mule! :D
10/30/2004 9:30 AM
Bruce /Mission Amps

For what it's worth, I just finished a new amp build yesterday that uses a pair of fixed bias KT88s...(similar to 6550s) and a 5AR4 rectifier.  
With a 4200 ohm OT, which is about the same size as a Super Reverb OT, power tranny is 200ma, B+ is 460vdc, tubes are idling at 46ma each, ...max clean output power into an 8 ohm dummy load @ 85Hz was 54 watts and I was expecting a little more.  
6L6s set up correctly at 35ma in the same amp, same test produced 46 watts.  
Sounds like the OT in this amp is at it's limit with that primary Zed and core size.  
10/31/2004 2:12 PM

Don't know if this helps but I use a couple modded 2x6550 amps for guitar. One is a Dynaco Mk 3 where the spec primary impedance is 4300 ohms. Another is an RCA SA-1000 which have a spec primary impedance of 5K ohms. I use them w/o feedback and they each put out about 70-75W before clipping.  
By the way, I came over to ampage to see if anybodys used RUBY 6550ASTR tubes? They are supposed to be remakes of GEs. The ones I have are real sensitive to vibration and make awful crackling noises when used on top of a cab. I just wondered if they are duds or if anyone else has had problems with them? I don't have any trouble with NOS Tung-Sols and NOS GEs in the same amp.
11/1/2004 8:06 AM
On all the 6550 amps I've worked on lately, I've had customer permission to replace the 6550's with JJ KT88's, and never had a complaint.  
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