SixAPre
- Thread starter RPMac
- Start date
This shouldn't be a problem.
-PB
-PB
If the filament is sufficiently out of tolerance, then perhaps that could cause an issue.
Taking some voltage measurements of the tube that doesn't work operating in the BeePre would tell us for sure.
Taking some voltage measurements of the tube that doesn't work operating in the BeePre would tell us for sure.
300B's
tube 1...Fil--4.23vdc; Plate--142vdc
tube 2...Fil--4.25vdc; Plate--144vdc
tube 1 was playing, then faded out as I was taking measurements.
Put the 300B's back in ParaBee's and I'm listening to them now.
6A3's in BeePre
tube 1...Fil--6.13vdc; Plate--101.2vdc
tube 2...Fil--5.90vdc; Plate--98.8vdc
tube 1...Fil--4.23vdc; Plate--142vdc
tube 2...Fil--4.25vdc; Plate--144vdc
tube 1 was playing, then faded out as I was taking measurements.
Put the 300B's back in ParaBee's and I'm listening to them now.
6A3's in BeePre
tube 1...Fil--6.13vdc; Plate--101.2vdc
tube 2...Fil--5.90vdc; Plate--98.8vdc
Paul Joppa
Moderator
Yeah those cathodes are exhausted.
RPMac said:
Those 300B's aren't really drawing any plate current. What is your "POS OUT" voltage from each regulator board?
This is about what I'd expect from some well worn 300B's. They'll still work in a cathode bias power amp (kinda), but not in something as tightly regulated as the BeePre.
I'm assuming you mean the high voltage reg (ter.6--144.8vdc & ter.11--145.9vdc)
The Pos Out on the PSU board 10.07vdc & 10.00vdc
Measured with 6A3's in.
As for the 300B's, I guess that is what you get with a cheap "deal" on eBay.
In the BeePre, would a plate voltage much above ~100vdc or +110vdc indicate a tube that is expiring?
The Pos Out on the PSU board 10.07vdc & 10.00vdc
Measured with 6A3's in.
As for the 300B's, I guess that is what you get with a cheap "deal" on eBay.
In the BeePre, would a plate voltage much above ~100vdc or +110vdc indicate a tube that is expiring?
Paul Joppa
Moderator
I'd say "fading". It should still work until the plate voltage is around 135v - that leaves 10v compliance to the regulated 145v, so you still have 5v peak output available with a 5v margin for the C4S to function properly. So at maybe 130v I'd say "expiring".RPMac said:
RPMac said:
Yeah, a current source replaces a resistor (fixed, resistive impedance at the frequencies we care about), with an impedance that is much higher.
Since you're cool with Ohm's law, we'll look at the voltage amp stage in the defunct Foreplay III as an example:
The stock circuit drops about 3.3mA across a 22.1K resistor from a 150V power supply, so 75V appears on the plate of the first stage.
When we install a C4S in this circuit, the 22.1K resistor is removed and replaced with a current source set to about 3.3mA. Consequently, the same plate voltage appears, but the load impedance that that gain stage sees is now several million Ohms (instead of 22,100 Ohms).
It isn't necessary to understand the workings of the current source, but rather that it allows us to circumvent the restrictions of Ohm's Law a bit. If we wanted that same gain stage with a 1M plate load resistor, 75V on the plate, and 3.3mA of current, we would need a 3,200V power supply and a 1M 30W resistor. When you go through the analysis, you need hundreds of dollars worth of parts to build a 3.2kV power supply, then you need those resistors too (they are about $100 each), then the pin spacing on the 9 pin socket would cause a lot of arc over before the tube drew current, etc.
No- I've just started the build. Since some of these tubes take a long time in shipment, I want to reach a decision. My logic on the 6a3 after research here is that: a)there are claims that they are less microphonic b) you can get vintage NOS American made tubes for around $150 matched pair which (in theory) means improved sound. I guess the 6a3 filaments have higher resistance, so filament voltage is pretty acceptable at around 6.1VDC. My guess is that the preamp with 6a3's may have less overall gain, but nobody has complained. If I go with 300B's, NOS not available at my price range. Looking at buying stock EH 300B's from you (if available separately), TJ Mesh, Psvane TII (currently in my Paramount 1.1's- $400), or whatever else my research digs up. I'm looking for feedback because I'm hoping to "buy once".
That said, I'm all ears on whatever advice you might have
. Excited about hearing combination of BeePre>Paramount 1.1>soon to be purchased Fostex BK-16 folded horn kit with Fostex FE168 Sigma full range drivers. Thanks, Kip.............
That said, I'm all ears on whatever advice you might have
. Excited about hearing combination of BeePre>Paramount 1.1>soon to be purchased Fostex BK-16 folded horn kit with Fostex FE168 Sigma full range drivers. Thanks, Kip............. Paul Joppa
Moderator
Just to be clear, the BeePre was designed and "tuned" around the EH 300B. It is a fortuitous accident that a 6A3 will also work.
As Paul said, it is just fortuitous that the 6A3 will work in the BeePre. I would concentrate on the 300B & think of the 6A3 as a different 'flavour' you can try at some point.
It's a while since I had 6A3's in the BeePre, but I don't recall any noticeable difference in gain & no 'improvement' in sound, just a slightly different presentation.
Buying NOS tubes on Ebay is a crapshoot at the best of times. There are plenty of NOS tube dealers who know what they're doing & usually cost less than what you'll pay on Ebay. Just checked a few sites and NOS 6A3 go for $25-40 each.
It's a while since I had 6A3's in the BeePre, but I don't recall any noticeable difference in gain & no 'improvement' in sound, just a slightly different presentation.
Buying NOS tubes on Ebay is a crapshoot at the best of times. There are plenty of NOS tube dealers who know what they're doing & usually cost less than what you'll pay on Ebay. Just checked a few sites and NOS 6A3 go for $25-40 each.
Chris65:
What are you running in your Beepre? Any favorites?
I'm always interested to find better tube sources- if you have a moment, I'd appreciate it if you could direct me to some of the quality tube vendors you mentioned.
Of course, there's always this option:
http://www.ebay.com/itm/MP-of-Western-Electric-300B-Tube-Double-Square-Getter-NOS-NIB-Date-code-6539-/301635864396?hash=item463ae6074c:g:IfIAAOSw0vBUcAM5
WOW!
Thanks for your time, Kip..........
What are you running in your Beepre? Any favorites?
I'm always interested to find better tube sources- if you have a moment, I'd appreciate it if you could direct me to some of the quality tube vendors you mentioned.
Of course, there's always this option:
http://www.ebay.com/itm/MP-of-Western-Electric-300B-Tube-Double-Square-Getter-NOS-NIB-Date-code-6539-/301635864396?hash=item463ae6074c:g:IfIAAOSw0vBUcAM5
WOW!
Thanks for your time, Kip..........
