[Amps] GS35B Amps Common Problem / few more comments

Wed Nov 6 22:32:53 EST 2019

I think it might fit, just about, There is about 73mm clearance from the 
tops of the 8874's to the cabinet of the 78. The 8877 measures about 
43mm taller than the tiny 8874. so there would still be a good bit of 
clearance.

Reworking the plenum would seem a reasonable requirement. This could be 
made larger also by removing the stock blower and mounting something 
more appropriate on the rear of the cabinet. Suitably designing the 
plenum should allow for a 50mm x 50mm or similar sized opening. Even 
quite a large blower should not protrude above cabinet height. A 
toroidal filament transformer might go where the removed blower was and 
allow nice short leads.

The HT is woefully short however but there seems enough current for 
overall power. People who have done the conversion to 3CX800's say they 
can get plenty of power so perhaps a doubler from 2000 to 4000 volts at 
appropriately reduced current might not be such a bad compromise.

However this is all quite a bit of work for a tube which, yet again, is 
becoming somewhat precious. Replacement 8874's can still be had, just, 
at around US$1500 for the 3, 3CX800's at around US$1000 a pair and 
8877's at circa US$500-800 (*) depending on source. Not outrageous but 
not pocket fluff either. Still there are no tubes which can achieve this 
degree of performance and likely available for the next 50 years at 
US$50 a piece (**).

The 3CX3000A7 is indeed an interesting choice though I think that really 
is too big for the 76/78 chassis without a top hat. Given how many 76/78 
amplifiers are out there that could be otherwise consigned to the B.E.R. 
pile it might not be such a big deal. Taller custom front panel and a 
taller custom wrap around top cover, an extra 100mm might be enough. Re 
designed plenum and external blower. Voltage doubler on the power supply 
which might be sufficient power for 1500 watts SSB service or a custom 
transformer to take account of the increased space from the taller cabinet.

It might be easier than a full home brew since a lot of the parts are in 
place but still probably beyond the average US extra class ham these days.

I have a Henry 2k console that is a bit of a semi basket case. The 
3CX3000A7 might be a tempting retrofit. It is the Classic Export version 
so has the super HD transformer. But at 9 inches high the Henry is still 
too short for such a tube. The cabinet butchery required would seem a 
bit extreme.

Martin, HS0ZED

(*) Based on prices I have been able to source, YMMV.
(**) Not that I've been able to identify, YMMV.

On 07/11/2019 08:46, Alek Petkovic wrote:
> A taller cabinet and I would suggest a heftier blower.
>
> 73, Alek.
> VK6APK
>
> On 7/11/2019 5:48 AM, Steve London wrote:
>> Okay....I'll be stupid....
>>
>> Besides the filament voltage and socket, what else would it take to 
>> convert something like a 3 tube Alpha 76 to a single 8877 ? Seems 
>> like the output Z is in the right ballpark.
>>
>> 73,
>> Steve, N2IC
>>
>> On 11/06/2019 02:26 PM, Jim wrote:
>>> Agreed! 8877 is one the the BEST tubes for ham radio.
>>>
>>> LOTS of them out there and available. Hams bought many of them for 
>>> spares that never got used; Because the 8877 was such a robust tube. 
>>> Most 8877 tubes got babied... Except for contesting... But there is 
>>> testimonial from K3LR's large and busy contest station below. So 
>>> even in a contesting environment, the 8877 goes STRONG!
>>>
>>> 8877/3CX1500A7 hard to beat!
>>>
>>> Thanks
>>> 73
>>> Jim W7RY
>>>
>>> On 11/5/2019 8:34 PM, Tim Duffy wrote:
>>>> Hello Jim,
>>>>
>>>> I agree - the 8877 - 3CX1500 is a VERY good Ham Radio tube. I have 
>>>> several in service here at K3LR that are 30 years plus years old.
>>>> 18 amplifiers here using 8877s that make 1500 watts output RTTY 
>>>> with high TX duty cycle - no problem - all the way to 220 MHz!
>>>>
>>>> The 8877 is an excellent engineered tube for amateur radio - easy 
>>>> to build amplifiers with the 3CX1500A7.
>>>>
>>>> 73
>>>> Tim K3LR
>>>>
>>>> -----Original Message-----
>>>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of MU 
>>>> 4CX250B
>>>> Sent: Tuesday, November 5, 2019 9:12 PM
>>>> To: Carl
>>>> Cc: amps at contesting.com
>>>> Subject: Re: [Amps] GS35B Amps Common Problem / few more comments
>>>>
>>>> You may be a bit harsh, Carl!  I’ve lost two 8877s  in the past half
>>>> century or so, and it was my fault both times. The first was when I
>>>> stupidly let the filament voltage drop way below specs, and the second
>>>> was when the blower on my Alpha 9500 failed and I didn’t notice for
>>>> two weeks. My workhorse homebrew amp, built in the 1970s, has an Eimac
>>>> 8877 with a 1978 date code. Still full output, 4300V on the anode.
>>>> 73,
>>>> Jim w8zr
>>>>
>>>> Sent from my iPhone
>>>>
>>>>> The 8877 is a fragile short life wannabee befitting an oxide 
>>>>> cathode tube
>>>>>
>>>>>
>>>>> Carl
>>>>>
>>>>>
>>>> _______________________________________________
>>>> Amps mailing list
>>>> Amps at contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>
>>>> _______________________________________________
>>>> Amps mailing list
>>>> Amps at contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>