300B GSG Mods

2nd harmonics for the masses

300B GSG Mods

Postby Shannon Parks » Thu Oct 11, 2007 6:18 am

1) Score some EH 300Bs from eBay for $106.
2) Wire in two UX4 sockets.
3) Change R19 & R20 to 1.5 ohms, 5W (e.g. Mouser pn# 594-AC05W1R500J).
4) Enjoy!

While the GSG won't cause your 300Bs to break a sweat, the sound is excellent. At these operating voltages, they are more linear than the 6B4s I've tested, so you end up getting another half a watt output or so at the same THD levels.
Last edited by Shannon Parks on Sat Jun 04, 2011 9:37 am, edited 1 time in total.
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Postby EWBrown » Thu Oct 11, 2007 7:38 am

If you replace the 6AX5 with SS rect (easiest mod is to solder in some uF4007s to the appropriate pins in an empty octal tube base), you should kick the voltage up by another 20-25VDC or so. WIth 300Bs, every little bit of B+ helps...

I did some voltage measurements on the still-in-process "300B on an ST70 chassis project", getting 455V B+, 435 VDC on the 300B plates, and 72-73VDC across the 1K cathode resistors 2 2K 10W in parallel, each).
A quick SE CAD simulation indicates this should be good for just a little short of 7 WPC.

This was strictly an "idle current" test, as I have built the SRPP driver board, but haven't yet installed it. The OPTs are some "Transcendar" 15 Watts, 3K primary 300B SET, that I scored on E-bay last year. Their DCR is about 280 ohms, so it appears that Ohm's Law still holds true Yellow_Light_Colorz_PDT_08

AES claimed that the socket and turret board that I bought and used was appropriately sized for an ST-70. It isn't, and the "ST70" board in the 2007 catalog is still the wrong size, too...

I'll need to do some mechanical wizardry / hackery to make it fit, and not look like a totally sloshed Homer Simpson built it.

I powered the 300B filaments with 5VAC, 2A trannies, one for each tube. Along with a 50 ohm WW "balance" pot. This gets all the 60 Hz hum out, I do get a tiny bit of 120Hz ripple. (Just from the 300Bs, with no other signal input).

Note: there is no electrical connection between the circuitry and the chassis at this time, except for the OPT secondary "cold" end. I am going to experiment with what point on the chassis yields the least hum / noise.

Since this IS the get*set*go topic, I have finally fired up the 6B4G version, and it sounds great!

Some day, will be a "shootout" between it and the "fancied up" Bottlehead Paramour IIs. The BH amps do run the B+ voltage higher, and the split rail scheme puts around 320VDC from P to K (ok, F). But then, Bottlehead specifies 1100 ohms for the 2A3 cathode resistors.

The next G*S*G incarnation, for the second board, will use 6C4Cs s and some Magnequest RH-60 OPTs. I ran the SE CAD on this combo and it looks really good. Since the OPTs have a 5K primary, the power out is a tad less, but the 2nd harmonic distortion falls off fairly significantly. I could "cheat" and use the 16 ohm secondary tap, and make it look like a 2.5K OPT, for more power output and higher 2nd harmonic content.

Then there are some 6BL7s t experiment with.... And 6CK4s, etc... Yellow_Light_Colorz_PDT_07


/ed B in NH
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SS retification in get set go

Postby lth1 » Mon Oct 29, 2007 4:48 pm

Ed,

"If you replace the 6AX5 with SS rect (easiest mod is to solder in some uF4007s to the appropriate pins in an empty octal tube base), you should kick the voltage up by another 20-25VDC or so. WIth 300Bs, every little bit of B+ helps... "

Now that you have gotten my interest, what are the appropriate pins?

Cordially,
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Postby EWBrown » Tue Oct 30, 2007 7:07 am

Lets see, I just printed out the 6AX5GT's data sheet....

Solder one uF4007 or uF5408 anode each to pins 3 and 5, and then tie the two cathodes together and solder them to pin 8. You will have a b+ increase of approximately 45 VDC, and somewhat higher power output.

It may be a good idea to return to the originally spec'ed 1100 ohms for the 6B4Gs' cathode resistors with the higher B+, if you plan to stay with the SS rectifiers. With 330VDC B+, the 900 ohms performed better.

Do not connect anything to pins 2 and 7 (the filament).

HTH

/ed B in NH
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Postby ecir38 » Tue Feb 19, 2008 9:48 am

I am creating a cd for the G*S*G to put on file for future referance. Here is one file that will be on the cd of a modded schematic for the 300B version.

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Postby ecir38 » Fri Feb 22, 2008 6:18 pm

Pulled this quote from here http://www.diytube.com/phpBB2/viewtopic ... 5617#15617 Didn't want to hijack that thread and thought this topic would be better here.

TerrySmith wrote:My plans for a 300B version is use the 6123hs outputs, and for power a Heathkit AA100 pt, 375-0-375v, two 6.3v, and one 5v windings. For the rectifier I will rotate, cut&jumper the socket to use a 5AR4 or maybe a JAN-5R4 "tater masher", that would look cool!


Sounds like a great idea and would give the folks that use the 300B allot more options on the type of rectifier tube you could use for this setup. If I recall the 5AR4 is indirectly heated which should be a plus too.

What B+ voltage do you think this mod will make? After looking at the specs for this tube I think this would increase the voltages to similiar of a solid state rectifier but am not sure since I am still new to tube amp designs. Maybe with a bigger power tranformer.
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Postby Shannon Parks » Sat Feb 23, 2008 6:56 am

The 5AR4 would require a 5V filament supply and some hardware mods. Looks like you can rotate it one pin and cut and jumper to pin 2, as well as cuts from the 6SL7 filament circuit. This would be 300B & Sovtek 6B4 friendly (with the right power transformer), but not for old style 6B4Gs. Some rebiasing may be necessary.
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Postby Shannon Parks » Thu Oct 20, 2011 5:31 am

I did this data early this summer before I changed my max output triode testing methodology, so max output power in these tests is relative to 5%THD. I used my 3.5K primary connections, but I think you can use the 2.5K for a little more output power. A good percentage of the 2nd harmonics may actually be from the single stage 6SL7 which is swinging a lot of voltage for the 300B.

get*set*go 300B with 6AX5GT rectifier at 1W
get*set*go 300B with 6AX5GT rectifier at 5%THD+N
get*set*go 300B with 1N4007 rectifiers at 1W
get*set*go 300B with 1N4007 rectifiers at 5%THD+N

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Postby badassbob » Thu Oct 20, 2011 4:44 pm

With the voltage swings of the 6SL7...how much stress will it be under? Enough to have to replace it every so often, or will it "stay put" as it would with a 6B4G?
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Postby Shannon Parks » Thu Oct 20, 2011 5:01 pm

The 6SL7 won't be stressed at all, so you are A-OK. More swing just means less linearity, but it is still a less dominant part of the total distortion than the 300B's own distortion. And the drive capability is there, too.

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Re: 300B GSG Mods

Postby EWBrown » Sun Mar 30, 2014 8:07 pm

As mentioned in my other 6AV5GA postings, the 6AV5GA behaves more like a "small" 300B, than to a 2A3 or 6B4G. I confirmed with using SE CAD to compare actual observed voltages, current and output to 6B4G and 300B, the 6B4G was approx 10% different, the 300B was within 2% of the 6AV5GA under actual operating conditions.

Just keep the 6AV5GA's idle plate dissipation to a max of 20 Watts, plate current to no more than 65 mA max, and V P-K to 320VDC.

Yes, 20 Watts is correct, in Class A operation, the plate dissipation can be pushed well beyond the TV horizontal deflection 11 watts ratings, with no ill effects.
Tube Lab George has taken some 6AV5GAs up to nearly 40 Watts without the dreaded red-plate disease occurring, but I wouldn't recommend going this far O:)

Best to use the larger envelope 6AV5GAs, and Sylvania or Dumont brand are definitely the best for this application, some of the RCAs and GEs have wimpy,
undersized plate structures, more akin to those found in the smaller and somewhat weaker 6BQ6 or 6AU5. Those tubes don't like to be pushed too far beyond 11 W PD, 15 W is like skating on very thin ice... =:o

/ed B
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Re: 300B GSG Mods

Postby tuyen » Thu May 08, 2014 8:20 pm

Hi guys,

Loving the GSG!

As instructed, I replaced the R19, R20 with 1.5ohm 5W resistors. Fitted 4 pin sockets. Plugged in 300B tubes. Noticed that the amp has noticeably less gain compared to the 6b4g setup. Should this be expected?

I have a pair of KenRad 6A3 tubes too. Can I use them now I have UX4 sockets in? (Without changing back the .75ohm resistors)

cheers!
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Re: 300B GSG Mods

Postby TomMcNally » Fri May 09, 2014 9:30 am

hi tuyen -

I've built a bunch of get*set*go amps, and haven't really noticed any change in gain between
the 300B and 6B4G ... how drastic was it ? Are you basing it on what you hear and where the
knob is set on your preamp ?

No - don't use 6A3's with the 1.5 ohm, your filament voltage would be 5 volts not 6 (roughly)
it would be easy to use two .75 ohm resistors in series, and short one out from the bottom
of the board with a jumper for 6A3's, or use both in series (1.5 ohms) for 300B's

... tom
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Re: 300B GSG Mods

Postby Shannon Parks » Tue May 13, 2014 5:53 am

Take the voltages from the manual, Tuyen, and email them to me. Gain shouldn't be different.

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Re: 300B GSG Mods

Postby zman01 » Sat Jan 17, 2015 11:21 pm

Shannon,

Since 300B tubes would need higher B+ vs 6B4G, what if a builder chooses with a transformer with higher secondaries i.e. 360-0-360 vs PA-774 which is 330-0-330? Will the 6AX5GT rectifier be able to get the B+ to acceptable levels (380v-390v)? Any changes in the BOM for this?
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