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THE AM BULLETIN BOARD => Technical Forum => Topic started by: AJ1G on March 11, 2018, 01:52:41 AM

Title: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: AJ1G on March 11, 2018, 01:52:41 AM
Son Ben asked to borrow an old Newcomb E-75 PA amplifier I have here to use at a local bar to play vintage rock and roll vinyl through.  I pulled the amp out of storage and it powered up nicely, sounded very clean.  Am wondering if I might have to check the biasing on the Sovtek 6L6GCs I put in it years ago to replace the original 7027As.  Canít recall why I didnít stick with 7027s, probably was a cost issue.  The amp sounds great, but the 6L6GCs are showing a bit of orange color at idle, that doesnít get worse at full output.  After an hour burn in at full strap playing my Pandora Rockabilly Radio channel, I shot some temps with a IR temp scanner.  Max the temp I saw was about 435 degrees F on the upper end of the plates ( through the glass envelope).  The power tranny was at about 130 F.  Is it normal for 6L6GCs to show a little color at idle?  IIRC the amp is rated at about 45 Watts RMS.

Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: W3RSW on March 11, 2018, 05:52:23 AM
Bar music can stand much more class B than A so Iíd reset that bias again.  ;D
A little distortion for a cooler amp is a decent trade off.
Donít think Iíve ever seen orange in a 6L6GB or GC, but then Iíve never been a roadie.

The KT88ís in my Citation never run orange. Similar tube rated at 60 watts per channel under very heavy multiple feedback loops. They do show a little blue on the envelop interiors due to plate electron escape.

Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: AJ1G on March 11, 2018, 03:48:17 PM
Had a chat on 75 this morning on this subject with TimTron and Al, 1VTP, all agree that  pltes should not be showing any color.  During the chat I checked all the voltages of the amp, and everything is very close to the nominal numbers on the schematic affixed to the insoide of the chassis bottom cover - 587 VDC vs 590 at the 6L6GC plates, 399 VDC vs 400 on the screens, and -38VDC vs 40 at the grids from the fixed bias supply.  So bias is a LITTLE low, will try to bring it up a bit, no pot so will have to change or parallel resistors.  There are relatively cheap 7027out there, but from what I have read, most of them made offshore now are essentially repackaged/relabeled 6L6GCs.  Also read that the original 7027A was essentially a repackaged 6L6GC with modified wiring arrangement in the base to support higher plate voltages,  and slightly different base wiring with respect to the grid and screen elements (each go to 2 vs 1 pin).   That wasn't an issue for me as the amp had those pins already wired together below the socket. Going to see if I have other 6L6GC in my inventory. i did briefly sub in some original RCA 6L6Gs with the old school shouldered glass envelopes and they seemed to run cooler by IR scanner readings than the Sovteks, could not see if they were showing color as the envelopes have a gray mask on the inside of the envelope glass.

If the tubes are indeed electrically generally equivalent, I'm thinking that even new good quality 7027s may show color, so I'll need to tweak the bias up a bit.

Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: Opcom on March 11, 2018, 06:49:34 PM
The plate voltage is high for a 6L6GC. Screen is about maxed out. More bias should do it.

Get a handle on plate dissipation. Max is 30W. Try 20-25W per tube, consistent with low crossover distortion. You could try inserting a 1 Ohm resistor in series with the cathodes to read cathode current (or one in each cathode lead, or even use the plate leads, being careful.) and then adjust bias up to make the plate dissipation no more than 25W, and that should give a good margin for tube life and no color. The amp should already have enough negative feedback to avoid excessive crossover distortion.

so 590V on the plate, with 34mA per tube, the dissipation is 20W. 42mA would be 25W.

Screen wattage is 5W max, but better to keep it low also when the voltages are run up high. Under full signal conditions, you do not want the screen dissipation to be exceeded, or for them to get red hot. What Newcomb intended may be a different matter, wanting to squeeze every watt possible, and tubes were cheap then.

If you can adjust the bias separately, all the better. Make the resting current equal for both tubes, for better use of the output transformer. Of course the cathode resistor method also includes the screen current but it is usually small so it could be ignored in the bar application.

There are some old specs for the 6L6 (non-GC) at high voltages, making more than the traditional power, but they were dropped from most tube operating data and left for equipment manufacturers to figure out. The attachment has the 'official' 6L6GC data with plate curves including the higher voltages and G2=400V. Hope this might help in your high voltage application.

Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: Detroit47 on March 11, 2018, 11:59:07 PM
Hi Chris

Good QSO this morning.  I was thinking about putting 6550's in there. As I recall the have more plate dissipation and will plug in.
John N8QPC

Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: KD6VXI on March 12, 2018, 03:31:58 AM
6550s work.  And if you really want to boogie, kt88s and I believe the kt120 plug in as well.

I've used the 88s and the 6550s before.  I seem to recall having to play with the cathode resistor to get the idling current where I wanted....


Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: Opcom on March 12, 2018, 11:13:58 PM
watch the heater current on the 6550s.

Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: N1BCG on March 13, 2018, 09:06:24 PM
Just for fun...

As a "Jamerican" (American with a Jamaican spirit) I wanted to share that the term "Cool Runnings" is used for wishing those you care about "safe travels" before a journey.

Just love that expression...

And the original 1982 "Hot hot hot" by Arrow can't be beat!

Title: Re: Newcomb E-75 - Cool Runnings or Hot Hot Hot?
Post by: KB2WIG on March 17, 2018, 10:36:43 AM

17 years sgo today, I was drinking beer at Concy Joe's in Jensen Beach, listening to Irish music being played on steel drums by a Jamacan band. Erin go brah, mon.

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