Octals are designed for most tube amplifiers

However, there are some incompatible circuits, for example:

X Single-Ended Amplifiers (Fender Champ, 5F2, etc…)

X Amplifiers with a paraphase phase inverter (Fender 26, 5A3, 5B3, 5C3, Gibson GA 40, some Valco and Supro circuits, Benson Amplifiers…)

X Amplifiers that use tube cathode as signal input (Musicman HD series)

X Amplifiers that use EL84 power tubes (Vox AC15, AC30, DR Z MAZ 18, Top Hat Club Royale…)

X Amplifiers that use the 7591 octal power tube (Ampeg Jet, Premier 50, etc…)

If you have questions about an amplifier’s compatibility with Octals, please email us at:

info@verellendevices.com

DATA SHEET PDF

OCTAL FAQs

the Basics

  • Octal is a plug-and-play alternative for the power vacuum tube used in audio amplifiers. Unlike traditional vacuum tubes, it uses modern electronics to deliver consistent, high-quality sound with improved reliability, durability, and efficiency.

  • The Octal is designed for amps with a push/pull circuit (not single-ended or triode connected) and a standard 8-pin power tube (EL34, 6L6, KT series, 6V6, or similar common octal pinouts). Please read the warranty section in this FAQ.

  • Nearly all new vacuum tubes are made in just three places: Russia, China, and Slovakia. This limited production makes the supply chain fragile and prices more unpredictable. After they are made, vacuum tubes have to be sorted, matched, and tested for things like microphonics, transconductance, and stability. Over time, they wear out from heat, use, and vibration, so they need to be replaced regularly. Tubes also handle high temperatures well and sound great, and we hope they never go away — but as an alternative to counter some of these issues, we came up with the Octal.

  • Some tube retainers can be a little awkward. We recommend removing them if you want to prevent scratching your Octal. The Octal pins have had a pretty healthy grip in every amp socket tested thus far.

    Also amplifiers with a B+ plate supply voltage over 500V will need fan cooling. Most such amps already include fan cooling (Ampeg SVTCL , etc..), but we offer a fan kit for amps that run hot without a fan (Sunn Model T, Sovtek Mig 100 etc…)

More on Compatibility

  • Your amp's power tubes have one job: make your speaker move air so you can hear your guitar. Your guitar signal swings back and forth constantly, positive then negative then positive again, and the power tubes push that swinging signal into your speaker to recreate the sound.

    Single-ended amps (like the Fender Champ) use one set of tube(s) to handle that entire swing alone, positive and negative both.

    Push-pull amps split the job between two sets of tubes: one handles the positive half of the swing, then hands off to the other, which handles the negative half. They take turns, back and forth, thousands of times a second, like a two-person relay team instead of one runner doing the whole lap solo.

    About 95% of commercially available amps are push-pull, and that's what Octal is built for today. A single-ended version will be in development.

  • Possibly down the road, depending on demand.

  • The Octals need at least 1.05" from the tube socket center to any neighboring physical obstructions (other tubes, transformers, etc.). This spacing is compatible with the vast majority of amps.

    See the datasheet above for more detail on Octal dimensions

  • The Octal is a suitable replacement for all standard 8-pin tubes. It isn't designed to emulate any specific tube model, but rather to do a good job of safely making your speakers wiggle and maximizing the output potential of your amplifier.

  • If your amp uses a rare 8-pin tube with a different pinout, such as the 7591S, the standard Octal is not a fit — but there could be a modified version for such amps down the road if there's enough demand.

  • Not recommended. Early experiments with this have shown that the Octals can dominate the workload, unbalancing the power amplifier.

  • Octals are designed to work in both. There are lots of configurations out there, so we'll keep testing to find and expose any compatibility issues.

  • We've had reports of folks having success with their Octal-loaded tube amp using speaker attenuators, but we've also had reports of some folks experiencing strange sounds. So while we don't explicitly claim incompatibility, proceed-at-your-own-risk.

  • Early tests with a vintage Sunn Model T, which uses an ultralinear output transformer, have been great.

  • Yes. This first version of the Octal does present distortion issues when used with amps using a "paraphase" phase inverter circuit. Examples of this are early "tweed" Fender amps and a rare few modern boutique builds. There will be a future version to accommodate these circuits.

  • The Octal doesn't support the rare triode-connected configuration, but we will consider a compatible device option depending on demand.

  • We don’t recommend this. 1.) The amplifier will put out more than the half power you’re going for and 2.) The two Octals will work harder than they otherwise would have to.

Tone & Performance

  • We're recording as many comparison demos and reaction videos as we can, because honestly, a recording can only tell you so much — you really have to hear it in person.

    Our impression: the extra power tends to add more body to your tone, and the extra headroom makes loud playing feel less squashed and more open.

    Going into this project, we wondered how much of a tube amp's sound actually comes from the tube itself versus everything else in the circuit, especially the output transformer. Turns out tubes matter, but they're not the whole story — your signal still passes through that output transformer on its way to the speaker, and that transformer shapes your tone as much as the tubes do, maybe more.

    So here's the short version: swapping tubes for Octals is a similar-sized change to swapping one tube type for another, like putting KT88s in an amp built for EL34s. Your amp will still sound like your amp.

  • Likely yes, though different amplifiers have different power supplies and output transformers, and the Octal will respond to each a little differently. Thus far, we've seen clean power increases up to 50% in different amps.

  • Yes. If your amp experiences an increase in available power with Octals, your speakers may be subjected to that additional power, so suitable power-handling speakers will need to be used to ensure they aren't overloaded. Note that a "100 watt" amp pushed into full power actually puts more power into your speakers than 100 watts — for example, a common Marshall 1960 4x12 cabinet is rated for 300 watts and paired with a "100 watt" amp that puts out more than 100 watts as the output waveform distorts. We'd expect an Octal-loaded "100 watt" amp experiencing a 40% increase (around 140 clean watts) to still be a safe match for a 300 watt Marshall 1960 cab.

  • Octals are capable of delivering much more drive current than vacuum tubes. Combined with the fact that Octals don't need a hot heater filament, this allows us to drive more output power per socket than a standard tube, giving your amp's power transformer a little break. Other benefits include precise bias current that doesn't need adjustment, and precise transconductance on each Octal.

technical specs

  • No, the Octal is internally factory biased for safe operation and zero crossover distortion in a push-pull amplifier.

  • No

  • So far, the Octal has been stress-tested long-term in amps running plate voltages as high as 700 volts.

reliability & Testing

  • Unlike a vacuum tube, the Octal experiences no degradation with continued regular use. So long as it isn't physically damaged or exposed to extreme conditions, it should continue to work indefinitely.

    • We ran them clipping in a 700V supply amplifier for hours straight

    • Tested below freezing at 30°F and above ambient at 200°F

    • Power cycled 10,000 times

    • Idled for days on end

    • Tested extreme impedance mismatches

    • Drop tested at 8 feet.

    • Months of real-world touring

    • Year + of live and in-studio use

    Octals survived all of the above at safe operating temperature and maintaining a consistent idle current.

  • The Octal doesn't use a heated filament, but it still uses a significant amount of power to drive your amp, and that power handling generates heat — normal and part of safe, efficient operation. For comparison, Octals don't get hotter than 300°F, while vacuum tubes can reach 500°F. If your amp has a power supply voltage above 500V (examples: Ampeg SVT, Sunn Model T, Mesa Bass 400), we recommend using a cooling fan on your Octals — most such amps already come stock with one. If not, we offer a fan kit.

  • We're seeing great results with the Octal in the dozens of amps we've tested so far, and we're constantly testing more. We've designed the Octal to be compatible with all standard push-pull 8-pin amplifiers, but we'll never be able to directly test in the thousands of amplifier designs out there, nor test every variation, mod, or repair a given circuit may have seen over the years. So for all these reasons — just like a new set of tubes — though we can guarantee Octal functionality, we can't guarantee a specific sound in your specific amp. We'll continue testing as many amplifier circuits as possible and documenting demos and reactions to try and communicate general compatibility, along with firsthand user experiences.