300B - Push Pull or PSE to drive a commercial speaker

[arnoldc]

hi arnold,

how about your output stage B+ and output traffo specs? knowing those can help determine grid bias and drive requirements.

you do not have to ditch your lundhall if you want it...

Hi Tony,

I just sold my James 6113 and am considering a Tamura F-783 which is 5K and 100mA. I'm looking at 400V for the 300B for the B+

Another option I'm looking at 5842 for the 2nd differential stage.

 

[Eli Duttman]

Hi Tony,

I just sold my James 6113 and am considering a Tamura F-783 which is 5K and 100mA. I'm looking at 400V for the 300B for the B+

Another option I'm looking at 5842 for the 2nd differential stage.

Arnold,

Look at the 300B operating conditions chart. Assuming the Tamura F-783 is 5 KOhms, end to end, indicates operating a 330B with 400 V. on the plate, -84 V. on the grid, and I_B = 80 mA., for a 12.5 W. O/P. Unfortunately, the F-783 can't take 160 mA. (2 tubes) of idle current and the operating conditions rate to shorten tube service life.

IMO, you should use the chart to guide the selection of both O/P trafo characteristics and operating conditions.

 

[Bandersnatch]

hey-Hey!!!,
I don't feel like worrying about SE circuitry, but PP 300B can be done quite simply. Stay Class A, and run 7-10k a-a of load. There's lots of Iron that will stand a 300B idling in each half. 5 to 6k6 would also do OK, but you'd absolutely want to stay class A in order to stay away from running a 1k2-1k5 load when one side cuts off.

At this level, I'd trade a 20% power cut for better damping any day to run a higher numeric load.
cheers,
Douglas

 

[arnoldc]

Hi Eli, thank you for the reminder. That's the operating point I've chosen as well.

ps.

I'm not sure my hands are doing what my brain said, but the F-783 can take 250mA per side and 20mA of imbalance. I'm don't know why I typed 100mA!

 

[arnoldc]

Here's what I have so far-

The driver stage is 417A/5842 (I have a couple of them, and I like their tone), and as much as I would like to direct cuple to the next stage, I don't want a high negative supply for the succeeding 6BX7 differential pair.

The 6BX7 are running at 10mA @ due to the limit of the LL1660S/10mA.

If I can't get a 6BX7, I'd put in a 5687 in that position.

Comments? Positive or negative are more welcome

 

[Bandersnatch]

Here's what I have so far-

The driver stage is 417A/5842 (I have a couple of them, and I like their tone), and as much as I would like to direct cuple to the next stage, I don't want a high negative supply for the succeeding 6BX7 differential pair.

The 6BX7 are running at 10mA @ due to the limit of the LL1660S/10mA.

If I can't get a 6BX7, I'd put in a 5687 in that position.

Comments? Positive or negative are more welcome

Isn't that 10 mA spec. for SE use of that IT? I think one of the local chaps is using a minimum gap 1660 as a plate load on a differential stage at substantial current to drive 813's.
cheers,
Douglas

 

[TonyTecson]

you will need around -30volts of negative bias on the 6bx7...using a 5687 in this position, you will be running it at 6 watts, a bit too hot for that peanut tube.....

 

[arnoldc]

Hi Douglas,

I checked the LL1660S/10mA specs again, and it said Primary DC current for 0.9 Tesla for PP-PP is - and for SE-PP is 18mA.

How much current did your friend passed through on the 1660S?

Hi Tony,

Yes, my bad... the 6BX7 bias alone is alreadt -18V I did not mean to interchange from 6BX7 to 5687, so the operating point will change. Nobody has replied to me on the LF: 6BX7 on our local forum

 

[TonyTecson]

if you can find the 6BL7, that can also do in place of the 6BX7. otherwise, if octals can not be found locally, i suggest 12B4's...

 

[arnoldc]

Ok Tony, will take you on the 6BL7 reco and 12B4

 

[arnoldc]

I am disappointed that the Tamura F-783 being sold to me is over-priced. The brand new one from EIFL is US$265 only.

Anyway, my quote for the James 6233HS (100mA/side) arrived and I'm highly considering it for this project.

ps.

For those who might want to know, the 6233HS is NT$9000 for a pair. Shipping to the Philippines via EMS is NT$1480.

 

[adam2a3]

300B PP amp

Hi Arnold,
Long ago (like 10 yrs now) I designed a 30W PP 300B amplifier with regulated supplies which was published in VTV and became part of the SED technical library, (long gone) and while I am not recommending you build this particular design I do recommend you at least read the article which is on my site.

I don't recommend AB2 operation for any 300B, as the grids are rather easily over-dissipated, plus at 400V B+ and 120mA quiescent current you can get 30Wrms out with a primary PP of 3.8K and at less than 2% thd with no feedback. Any transformer in the range of 3K - 5K will provide good results.

I used fixed bias in all my designs and recommend that.

This design depends heavily on the quality of the regulated power supplies and was very carefully designed to provide the required drive on a +400V supply, unfortunately the LTP based driver stage has limited bandwidth when driving the miller capacitance of the 300B.

Lots of people have built these amplifiers, and in addition I sold a fair number of a simpler version of the 300B PP amplifier commercially as well.

Note that I have not found ac heating to be very acceptable long term in my commercial 300B based PP amplifiers, and ended up retrofitting at least 5 of these amplifiers (KTA3030) for dc heating at great expense. All subsequent units had dc heating. All would be well initially and then after a few weeks or months the tubes originally well matched would age and drift out of balance - hum would become intrusive. The procedure of setting the bias and then nulling the hum was tedious for the average user, and not as effective as expected. The JJ 300B is the only tube I have used that did not have this issue over the long term.

Here's the link: 300B Stereo Amplifier

Hello Kevin,
I recently (2013) built the 300B pp amp following your design and the sonic result is excellent. My amp has external power supplies (except a pair of 5VCT toroids) and I am using JJ300B's and HASHIMOTO 5K output transformers. I have also installed an 1-2 sec overall soft start and 1.5 minute delayed +400V start. Almost all caps are oil type inluding a huge MKV. Power supply is steel shielded while amp is aluminum. Umbilical has individually shielded cables and star ground in the amp box, including a ground lift (bridge etc). Hum and noise are simply inaudible, ear on the speakers.
I would like to ask you a couple of things.
What is the maximum safe plate current to adjust the JJ 300B's?
I am using a 500K dual pot for volume control in unbalanced mode. How do you adjust volume by turning a single control if amp is set for balanced input?
Any suggestions/improvements over the years? Thanks, greetings, adam2a3

 

[Ceglar]

Comments? Positive or negative are more welcome

Hi,

With respect to the output stage, I can recommend running them as a differential pair. In my experiments it was clearly better than either fixed or cathode bias in Class A, simply stick a CCS under the output pair.

The amplifier also used an interstage transformer with split secondary windings for the output stage bias, just as you propose. What you could do is sit the CCS on top of two paralleled pots, wipers to individual grid return windings.

The CCS keeps the voltage across the pots constant, and with some suitable value you can easily get the necessary bias volt adjustment to balance the idle DC currents. The pots are (to my visual) out of the AC signal current loop. I ran it this way for quite a long time and it was stable and sounded excellent.

The only other comment is if you want to go with AC filament supply, consider the 2.5vAc filament JJ 2A3-40. Super quiet with just a simple CT on the filament transformer, they bias up like a 300B. Something that you might not get with a 5vAC heated pair of tubes, but you might get lucky - for a while or otherwise.

In any event, enjoy it. Whatever you build will sound great!


Regards,
Shane

 

[TonyTecson]

even in single ended, ac heaters on the 2a3 gave a dead quiet amp...

 

[Ceglar]

easier to keep quiet than 300B thats for sure.