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Wednesday, April 9, 2014

Tube of the Month : The 801A


I already mentioned some favourite tubes in previous tube of the month posts. My favourite indirectly heated tube, favourite rectifier, etc. If I had to choose one very favourite tube of them all, it would of course be a thoriated tungsten filament directly heated triode. And it would clearly be the 801A.

Why the 801A? First I generally prefer to use directly heated triodes over indirectly heated ones, if I have the choice or if the budget of an amp or preamp allows for it. Directly heated triodes usually have very linear plate curves which enables low distortion sound without any crutches.

Within the direct heated triodes, I have a preference for those with thoriated tungsten filaments. To me they give a very transparent and high resolution sound if treated correctly. This means a very good, well filtered filament supply (DC of course). Any shortcut taken in the filament supply will become plainly audible in such tubes.

Oxide coated filament DHTs are a bit more forgiving in this respect. And lastly I have used 801As pretty much in any position of an amplification chain from the first stage of a phono preamp up to the output tube socket of a power amp where it can deliver almost 4W in Class A1 Single Ended configuration.

Like most DHTs the 801-A comes with an UX4 base and shares the same pinout with most other DHTs like 45, 2A3, 300B. The pinout is shown on the left. It requires 7.5V filament voltage at 1.25A. This pretty much excludes AC filament supply if you do not want to tolerate hum. I generally prefer a well filtered passive filament supply with LCL filters for the pure unmodulated sound they enable. The 801A has an amplification factor of 8 which makes it quite usable as driver and preamp tube and at least gives some gain. With a plate resistance of 5 kOhm it can be used with transformer coupling but needs good transformers with a healthy primary impedance. As output tube it is mostly used with 10kOhm load as suggested in the data sheet.
But I prefer higher load impedances (larger step down ratios) if it is used as power tube. I use 14k or more. The 801A has a reputation for a tendency to sound a bit thin and wiry. This is a sign of some impact of the filament supply or a too low load impedance. Properly used it sounds colourful with a full, well controlled and extended bass. The 801 (without A) is the predecessor but is interchangeable with the 801A in audio applications. The A version got some upgrades to improve usability in RF circuits. The military designation of the tube is VT-62. The 801A is an uprated variant of the 10Y. It has an improved maximum plate dissipation of 20W. As far as I now the 801A was only made with ST shape and straight sided glass. I never saw a globe version and I think it was never produced with globe glass.

As mentioned above a key strength of the 801A is it's exceptional linearity. These are the plate curves as plotted in the data sheet:

This set of curves extends far into Class A2 territory and the Class A1 linearity cannot really be judged well. Here a set of plate curves taken with a curve tracer from a 801a:

The plate curves are very evenly spaced which promises very low distortion if an operating point is chosen up in the straight part of the plate lines.

801As have been produced by many tube manufacturers. RCA made them over a long period of time in different variants and packaging styles. Let's start with some 801 and 801A in the famous world map box:

This is probably the most beautiful vacuum tube packaging ever made.

A nice large oversized box with thick padding for super secure shipping:

The tube is wrapped in a soft material together with a full spec sheet:

The spec sheet contains all you need to know to use the tube:

A 801A packed in similar style, but less soft inlay:

This tube bears both the 801 and 801A designation:

There had been even further oversized boxes from RCA:

This one has even thicker lining:

Back in those days, they knew how to pack and ship vacuum tubes. One down side of thoriated tungsten filaments is their fragility. They break easily especially after the tube has seen some use and the filaments got brittle.

Unfortunately they switched to more economic boxes later on:

Among the most beautiful RCA 801s are those with ceramic base and graphite plates:

I found these to work very well in line stages since they tend to have lower microphonics.

A different variant named RCA DeForest:

Another one:

A ceramic based 801 with the filament lit up:

For the 801A the ceramic base was dropped in favour of a brown micanol base. Instead of graphite, metal was used for the plate:

These came in different variants with different printings on the bases:

A close up showing the plate structure:

Next some versions from General Electric:

Also a nice oversize box with soft lining:

A beautiful micanol base:

Another version from GE:

This one has a graphite plate like the ceramic based RCA 801:

Some close ups showing the beautiful plate structure:

Hytron made a lot of 801A / VT62 tubes:

The brown color of the base can vary considerably between tubes:

A black base VT62 from Hytron:

They also made them with ceramic bases but unlike RCA with metal plate:

Quite interesting is this one:

Besides the 801A and military designation VT-62:

It also has '7A3' printed beneath the brand name:

7A3 would fit into the RETMA tube numbering system. The 7 for the filament voltage of 7.5V and 3 electrodes. But as far as I am aware 7A3 never was a RETMA registered number.

Sylvania also produced the 801A:

Another one:

An earlier Sylvania with ceramic base:

The top:

Taylor specialised mainly in transmitting tubes, so of course they also made the 801A/VT62:

The tube with the filament lit up:

Taylor also made a version with ceramic base.

Due to the fragility of the filament and the sensitive construction with tension springs, it is inevitable to get some damaged tubes which received rough handling during shipment. So I have some tubes for dissection. Here an unfortunate Taylor VT62 with the glass already removed:

Without the glass the internal structures can be seen more clearly:

The holder of the getter material:

The anode connection is wired through the side of the glass stem at the bottom to minimise inter electrode capacitances for RF operation:

The top:

As can be seen in the photo above, one of the filament tension springs is gone. Whenever you receive a tube with one or both of these gone, it is a dead tube and the filament is ripped.

A segment of the filament:

Close up:

The plate:

The top mica/ceramic support:

The grid:

A close up of the corner showing how the grid wires are welded to the support rods:

In the article about the 841, I already showed this photo of a 801A grid in comparison to the grid of a 841:

The 841 is basically a higher mu variant of the 801. The higher amplification factor is achieved by a different grid spacing (801 on top, 841 below).

The bottom ceramic support:

As mentioned at the beginning, the 801 and 801A were mostly produced with ST shape glass with the only exception of some variants with straight sided glass:

Beautiful white ceramic base and graphite plate:

No mica used inside, only ceramic to align the electrodes

I have seen such straight sided 801s from Amperex, Sylvania, Westinghouse and United Electric. Here one with the filament lit up:

A close up shows the glowing filament and grid wires:

Unfortunately these straight sided 801s seem to be very fragile. I have several with faults. Mostly open connections to the electrodes.

As mentioned at the beginning, I have used this tube already extensively. It works brilliantly as an alternative to the 10Y in the 10Y line stage or in the DHT phono preamplifier. It also serves well as a driver for example in 45 amps, 211 amps or in my latest 300B mono blocks. And of course as output tube as in the 801A/801A monos:

Best regards



  1. Have you ever built a PP amp? I know you like the 801A and I am slightly to hear what a 801A PP amp might sound like compared to a SET 801A.

    1. Hi Josh!

      I did build PP amps, but not with the 801A. With its high plate resistance the 801A is not really usable for Class A PP

      Best regards


  2. Lovely photography as usual Thomas. Thanks!

  3. Beautiful photos esp. showing the innards of the 801A !!!
    Thank you!


  4. Hi Thomas,
    There seems 2 different plate varieties. Can you kindly comment on the sound of each? Cheers. andrew AT perceive.nz

    1. Hi! I didn't write about the sound differences because that heavily depends on the amplifier. A subjective description of the sound of a specific tube in a specific amp does not guarantee that another person will experience it the same way in different amps. I know people keep talking about the sound of tubes but it is always the entire amp which is responsible for the sound.

  5. Hello Thomas,

    Long time no see ;) I love your website and the idea for the sakuma style 801A amps!


  6. Hi Thomas,

    If you don't mind my asking, about how many watts of A1 output are you getting with the 801A paired with the Lundahl 2735B 16K:8ohm?

    I am considering my first 801A design and weighing an A1 vs A2 bias point.

    Thank you!

    1. Hi! This depends on various factors pls check data sheets of the tube and transformer. In any case it will be around 3-3.5W if you are worried about 0.5W plus or minus this is probably not the right tube ;-)

    2. No concern here, my speakers are 6ohms 98dB/W :D from my calculations, I am seeing about 3.2W into a 16K load, just wanted to be sure I wasn't overlooking something.

      Thanks for getting back to me!

  7. Hi Thomas,

    Looking at the construction similarity between 10Y and 801A,I don't understand how 10Y can only be 10 watts and 801A twice that, 20 watts. Was 10Y more conservatively rated ?, or perhaps the plate is thicker ?. Also is there a construction difference which would explain the higher voltage rating.

    Best wishes

  8. Hi Thomas-I have 10's, VT-25A's, and RCA graphite plate 801ST, but the stright-sided Sylvania graphite-plate 801 seems to have more gain than any of the other 10 variants. Have you noticed this? Regards-Bill

    1. Have you measured that? How mucho is the difference?

    2. I haven't measured it, but I hear it. I use the 10 on the top of a bi-amped setup, and compared to my other 10's, I have to attenuate quite a bit more on the 10 amp (with Sylvania straight side 801) to match the bass amp.

    3. There can always be slight differences in gain between different tubes. If they are significant it is important to measure what is going on. This also depends on your amp. When the emission of a tube drops due to age it's gain can also drop (depending on the circuit ir is used in)